I'll back this horse. What stuck out most from what was said so far was that he was doing the same split. Uh, no. I'm a VERY firm believer that what works for one may not work for another. He can do the same split all he wants, but if he isn't doing what his body NEEDS in order to grow, then he's ... well... right where he is right now!
Instead of jumping into a bottle and throwin the Camaro fund out the window in lieu of some get-me-bigga pills, here's my suggestions:
1. Eat ... smarter. If he's eating quality food and in a caloric excess, then think about timing. WHEN is he eating those meals? Is he eating on a schedule? Is he eating for energy pre-w/o? Is he eating to RECOVER??? You have to fuel the fire, but to me, the most important fueling is done AFTER the gym, when the muscles are in dire need of repair.
2. Modify the workout plan, over and over again. So what if he's not doing the same as you? That might be a good thing, for both of you! Push hard, up the weights, break from what you've been doing. If he hasn't had a routine change in 4-6 weeks, NOW IS THE TIME!! Shock those muscles into new growth. Keep them guessing. And do it over and over and over again! If the program that worked for you 3 months ago is what you're using right now, I can guarantee it's not working anymore. You have GOT to keep it fresh. You want your body to change? Change what you do to it. Plain and simple.
3. Sleep. Rest. Take a nap. Whatever. Too many people ignore the rest aspect of training. This is when you grow. It's not just about sitting down to watch the tv and your biceps get heyoogeness after the gym, it's about sleeping and total rest. Ask any "stage walker" about their serious training time and I bet a lot of them will tell you that they take a nap during the day, if not two naps!
4. Nobody mentioned cardio here, so I will. When is he doing cardio? Is he doing any? What are the number of reps that you are doing? If you're looking to gain in size, doing a ton of cardio at bad times is counterintuitive. Do fasted cardio first thing in the morning, or do about 20 minutes after a good heavy lift. Otherwise, catabolism, baby! You'll be wasting away your gains by running/jogging/whatever-ing them off. Limit cardio, and if ya don't necessarily need it, I say skip it! Also, watch the time in the gym. Overtraining is also a huge killer for maximum gains. Limit your time in there. Get in, get work done, get out. Don't work everything for 5 sets in a single day. Go in and do back one day, 5-6 exercises, 3-4 HEAVY sets each, and GTFO!!! Go home, eat, sleep.
Hope that helped.