So charge him an invocation and let him (q.v. Agonising Blast). Or make it an alternate class feature (add PB instead of Str / Dex mod - but not both). You know, the old warrior may no longer have the strength he once did but he sure knows how to land a blow!
IMHO it doesn't matter if they're training wheels or not; what matters is if the players are having fun. It seems that this player is not.
It's absolutely fine to define the tiers as Training 1-4, regional heroes 5-10, and national heroes 11+. Yes, it's a higher power game, but if it's fun it...
How would you model the war flail in D&D?
I'm thinking d10 or d12 damage (because, you know, the damage is swingy), Reach, Bludgeoning or Piercing damage (depending upon whether the flail is spiked or not) and eligible for the Polearm Mastery feat.
That's what's called a plot hook. You can have the low-level PCs encounter more powerful adventuring parties early on. Maybe the more powerful parties hire the PCs to do something. Maybe the PCs seek out the powerful adventurers for a Raise Dead / Resurrection / Legend Lore / other powerful...
When I was playing 3E the low teen levels was when it all came together. Of course, multi-classing was rampant and 2-3 classes plus a prestige class was the norm. Rock on the Ftr 2 / Kt 2 / Paladin 4 / Divine Crusader, the Ftr 4 / Mk (you'd be surprised how many Monk abilities work in armour)...
If an adventure depends upon the use of a particular spell, make sure you provide that spell on a scroll or other device within the adventure. Preferably more than once.
Tod Cutler (weapon-maker) and Matt Easton (HEMA tutor) are playing with darts.
So, 1d3 + Str for the small ones (plumbata), d4 for the medium ones, and d6 for the big ones? And roll damage with Disadvantage if the target wears metal armour?
Not really. It was more the fighter types going in first. Either entering the room / crypt / whatever or - being strong - lifting the sarcophagus lid or seal or breaking the door etc.