Maybe he's being forced to be there, in which case I don't know of anything you can do except give him his space (and maybe have some sympathy for the poor guy. Don't you think it would suck to be forced to play some game you're not even interested in for weeks on end?)
Maybe he's shy! It can be hard to speak up in roleplaying games, especially when you're used to not speaking up. Even as a veteran roleplayer, I still have moments in games when I'm timid and have a hard time getting a word in edgewise. What generally helps me get back into the flow of things is when the GM or the other players prompt me by asking me directly for my input. For instance, when the party's trying to decide what to do: "Hey, what do you think? Should we stay on the forest path, or go check out that noise we heard?"
I've had success helping fellow players find their voice as well by inviting them to come along with me to scout ahead or investigate something without the rest of the party - I did this a couple times recently in a one-shot D&D game where I was one of the few experienced roleplayers amidst a group of newbies. I wanted to help give spotlight time to some of the players who weren't speaking up as much, so I suggested that the party ranger help me investigate around town while everyone else was busy at the tavern. Once the focus was on us, I made some suggestions but let him call the shots and take the lead. He took to it quickly! I also recruited the party rogue for some late-night breaking and entering, which led to some encounters where she played an important role and got to make some tactical decisions for the whole party.
(If you offer something like this and he doesn't bite, I wouldn't force it. The point is to play a supporting role to the other player's character and throw them opportunities to contribute in interesting ways, not to pressure them into an uncomfortable situation.)
Other than that, maybe you can find an opportunity to ask him what his favorite part of the game so far was, or what sorts of things he's looking forward to or hopes will happen. You can pass that information along to the DM, so they can use it to make the game more fun for him. When I run games, I try to get feedback from my players about how things are going, and I usually find that people are more comfortable talking concretely about scenes that they thought were cool, or plans they have for dealing with the in-game situation, than talking abstractly about general game qualities.
Having said that, just asking "hey man, what's up? You don't seem like you're having fun" seems to me like it's worth a shot. Maybe he'll just tell you directly what's wrong.