I'm not 100% on this, but I've heard that if you can free up some free test it could help with gains.
He should get some bloodwork done, take a look at where his free test is sitting, and maybe do something about that. He should make sure his estrogen is within range, as well.
Other than that... it might just be genetic. He might just not have that "gene" to gain mass. Maybe naturally high myostatin?
I'd love to hear what others have to say on the subject, I've wondered about this myself. I'm not a huge guy and I've been trying different combos of gear over the past 3 years or so. I've always considered myself as a "poor responder" as well.