Depends on how drastic his weight loss was. Ive seen people drop 20 pounds a month for 3 months and be fine. But f he dropped that 100 over a summer or something he could be in trouble.
Does he have enough extra skin to fan himself when it gets hot out? If so surgery is neccesary.
I had success, by accident, of going back in weight then slowly dieting down.
Other than that putting on muscle should fill in the gap and also massage, deep tissue massage will help with any scar tissue that is prohibiting the healing of that streched skin. Along with increasing your muscle size.