Should've suggested he try dipping into another class a bit and he might have really enjoyed it, melee sorcerers were okay with a splash of a martial class. My second character in 3.0 was a human who split his level advancements between barbarian and sorcerer, with the sorcerer part pretty much identical to your son's early on. Actually worked tolerably well for the nine levels the campaign lasted. That was partly due to us rolling stats and my having a daft spread with something like 17, 16, 16, 15, 14, 5 to work with, but the two classes were pretty synergistic even without that.
Didn't hurt that I had a lot of fun making up absurd nonsense whenever called on to make an arcana check with that 5 Intelligence.
I played one of the 3e era computer games, and made a wizard with one fighter level, and it was absolutely playing the game on easy mode. He was like a mediocre fighter that, when cornered, would simply incinerate every other mamajama in the room. This was before the Eldritch Knight was even a concept, just old-fashioned hacking of unanticipated combinations. A fireball, a reach polearm, and a fistful of attacks of opportunity. Plenty of people back then would have told you it was a Bad Idea, and praised the coming of the Eldritch Knight as a Much Needed Patch, but in that context, a very grindy CRPG with lots of undead foes, what I made was purely and beautifully optimal.
I played in a Rifts game, and I wanted to be a Frost Giant Headhunter. I'm not really into the stereotype of the big, strong stupid guy, and Algor aren't stupid, but wouldn't you know? I.Q. 6 (equivalent to about the same Intelligence score in D&D). Oh, well. Because of his bulk, he wasn't able to buy really good armor in his size, and had stick with what he started with, but he could hand carry some decent energy cannons and stuff. There was pretty much no synergy going for him, and while he was a little bit tougher than a human in good gear, he certainly wasn't equal to some of the stronger O.C.C.s and R.C.C.s But I had fun playing him, he was just a very straightforward guy who could bring the hurt. And there was this one battle where he just wasn't doing a whole lot, so in boredom, I had him switch from his usual energy cannon to some kind of pulse weapon. And then I rolled a 20, did double damage, and completely took the head off some mech we were fighting, and my guy became a party mascot. "What the hell did you just use?" some of the other players wanted to know. In truth, the weapon was almost identical in damage capability as his usual one, but it created a moment. So, yeah, a frost giant knucklehead playing a pretty poor species/class combination, rocking mediocre scavenged gear. But he was... sufficient.
So a "bad character" can be pretty context dependent.
In general, I blame the player. If someone is willing to read a little bit, and get a little feedback from the group, they should be able to make at least a mediocre, sufficient character with some expected capabilities and a little bit of flair or personality. But sometimes it's tempting to blame the player when the problem is the group... they don't "make room" for someone's perfectly interesting build or motivations or adventuring hooks.
I remember one time I was playing Fantasy Hero, and for my character, I decided to make a carnival juggler turned knife fighter. I wanted to try my hand at a basically "evil" character but one with the motivations to work with a normal adventuring group. His backstory was that he was imprisoned and sold into slavery, and he escaped with his buddy, a gladiator in training (one of the other PCs), and they became this sort of Fafhrd and Grey Mouser type duo. Anyway, most of the beginning went pretty well, and we got deep into the dungeon, when I began having a problem. One of the other players made a thief type, but their concept was this seductive spy character. Anyway, his character kept trying to solve disagreements with mine about strategies by trying to "vamp" him and misdirect. The final straw was when she stole some of his gear. This turned into a kind of ongoing spy-versus-spy dynamic within the group. I tried to sort of vaguely appeal to the group to stop his character from harassing mine, but no one seemed to care, so I decided this must be a fair game situation. So when his character blundered into a stun trap, after disagreeing with mine about a scouting strategy, my character took his almost-comatose rival and led her back to a disintegrating trap, where he was contemplating whether the head or the hand should go in... when he was caught in this awkward, compromising position by the rest of the party. At that point the party war-priest turned on him, the fire warlock said, "Wait, we need him alive. He's the only scout we have!" and it nearly turned into a party civil war. It was eventually ironed out and the game continued. Two of the players were very perturbed, and put the blame on me for playing an "evil" character, but I always felt the problem was mainly caused by the other player becoming adversarial toward a party member and thinking they could just keep annoying and humiliating them without repercussions. I mean, if the paladin caught a thief's hand in their adventuring pack, I feel like that could be an on-the-spot execution.
In any case, I had what I thought was a cool character, and enjoyed the first part of the game, but when put in the mix with this silly seductress-spy-thief and the hair-trigger war-priest, it just went sideways really fast. My character was too ruthless, but their characters were not willing to practice "honor among thieves." So in retrospect, "playful hijinx" was accepted with the group as a whole, but not "seriously committing to the mind-set of a professional adventurer, even if that means treating childish so-called allies as a liability." But both realms of behavior seem pretty acceptable within the concept of the game world as a whole. It seems like the GM could have, should have, been more of a referee on how the group was supposed to relate to each other. But the GM isn't ultimately responsible for the actions of the players. I was a little perturbed by the tacit acceptance of PvP behavior... but other players affronted when I took the PvP flag as being on, and engaged in what they considered unacceptable escalation.
Despite having something of a Session Zero at the beginning, and creating a web of PC relationships, we still didn't manage to hammer out the group dynamics enough to survive ourselves.
