Tell him if he has fast metabloism and continues to do alot of cardio or "mountain bicking" he is gonna burn alot of calories. To compesnate he would have to eat a ton of calories and sleep over 10 hours every night.
It is good that he remains active. He is probably quite healthy. When we try to change our own body's ideal comfortable weight it is always an uphill struggle.
Example: I gained 15 pounds of solid muscle after 3 months of SOLID training and watching my diet like a hawk.
Within 2 weeks of going back to my old lifestyle I lost 10 pounds. It is a battle to change your genetically predisposed weight and it requires vigilent maintenance or you will revert back to square one if you slack off.