Wow! That is one awesome chinstrap you got going. To answer your question, the only thing you can do to fill it in is give it time. Your way too young to use any kind of "meds" (rogaine) and it won't actually give you new growth, but just speed up the growth of what you have. You can do a supplement of vitamins, but I'm not sure how much that would help? I say just give it time. It will come in with time as you get older.

Beard on!