Why would he be worried about losing muscle if he's bulking up? The whole point of bulking is adding MORE muscle.
As for tips, what is he eating? Whatever it is, it isn't enough. From his height / weight, he must be a basketball player. In which case, he's probably doing a ton of cardio between practice and games. So he's going to need to up his calories even more to make up for all that extra activity.
What's his current diet / workout look like?