Well, I had BAD thinning on phera, and all came back.. I guess it depends on the individual.
I think it really depends on the individual. If it doesn't come back, your screwed. Hair growth isn't easy. It's better to prevent loss than to try to grow it back.
Lets say he did shed, and it didn't come back. His only options to look "young" again if he were in his mid 20's would be to grow it back or hair transplant. Regrowing hair with minoxidil works, but it will never be the same. If it did work well, we wouldn't have so many people with receding hairlines.
I would opt to get some aggressive topical treatment. Check out BigV's Hairloss thread. It should help you out. I would recommend spirolactone, ketoconozole 2%, Alezeic Acid, and Minoxidil to try to combat the shedding. Try it seperate from propecia. If the shedding stops or slows down it's a good sign. Minoxidil might make you shed hairs for a while. (The hairs that were going to fall out anyway) Then if all else fails propecia is your only bet. Hair prevention over hair growth. Unless you have 20g to drop down on hair transplant, which will only save you for a few years.