I've been working out for years. As a matter of fact, if I wear tight fitting clothing then I overhear people saying i'm on roids when i'm out in public or in the gym. I've never taken any of that shyt and never will, but random people swear i'm on some shyt.
Now that being said, you don't lose a ton of muscle mass while you're still youngish unless there's something up.
Three possible reasons:
1) He's fasting/starving himself for some reason.
2) He's sick.
3) He was on some shyt and is no longer.