15 is fine for creatine, but remember one thing. Creatine loses it's effectiveness if taken in LONG periods of time (over 3-4 months), and when he comes off he'll lose a good bit of endurance and strength. The body quits making it's own creatine, and it'll take him a good 2-4 weeks to feel normal again...
Been there and done thatw ith Creatine since 1997. Not worth it to me. Save your money and keep stuffing food in his mouth. Thats all he needs.