As many have said, this isn't necesarilly a problem player, just one who'se very clearly a lot more casual than you. That's maybe an issue when everyone else is trying to get more deeply involved, but not a critical failure.
You do, however, risk a nasty loop here. I have one player who has a habit of turning up for a session and then saying part the way through that he has to leave early: as in, not "can we stop at ten exactly cause I have a bus to catch" but "I'm going to the pub at 9pm so I'm leaving the game half way through". This means he's forever missing end of session cliffhangers and I can't afford to write heavilly-him-oriented plot threads because the odds are high he'll be away and make it awkward: this means he's going to feel less attached to the plot, so hey, it's not a big deal if he says yes to that pub visit next Thursday, right?.....
In the example I gave tehre's some player tension because it's becoming so frequent that it's bugging the other PCs as well. IN your case, though, do the other players mind that he isn't there at the end or does many online side-quests? If they're happy with him just doing the Minimum Contractually Obliged RP Time, then it's not really a problem IMHO.
THe miscommunications are perhaps more annoying for all, but this is a generic Person Problem and not a uniquely D&D one: you just need to be careful how you word things, explicitly tell him "do not assume X unless I say Y", and just generally be as clear as possible. If the rest of the players aren't getting confused, it's clear the problem is with him, not you, but there's no nice way to say "You follow instructions as well as a pet rock", so you need to do your best to stop it happening. :>
And re: his curtness when you asked for his help.... well, he does have a point. If you're not the GM of a game, you are in no way obligated to have the same investment in a game as it's GM does. Some of my PCs contribute to our campaign wiki, talk about D&D with me almost every day on IM, look up new feats/spells/etc at home.... and others basically turn up at sessions, do their thing, and go home. This gets annoying when it leads to extra work (i.e. I write plot catchups online for my group, but several of themd on't read them and instead have to be told mid-game when they forget who X NPX is that they met a few sessions ago) but that's, frankly, just the nature of some players.
I'd have a word with him and say that, while all the other players are into these extra things, he doesn't seem to be as engaged with your game, and you'd like to know if there's anything you can do to help involve him more. Don't phrase it like you're accusing him of something bad: make it clear you'd be happier if you could make him happier with the game. He might simply say "I don't have the time/inclination to play beyond the X hours our normal session lasts": in which case, you just have to build your game around that. If he isn't actually bugging the other players than chucking him might cause more strife in the group than it's worth.