Forgive me for commenting as I haven't have top surgery with him, but I plan to in the future.
As to your first question, I believe the $500 non-refundable fee will go towards your total amount for top surgery so if you are getting the D.I. for $6900, the $500 will go towards that.
As for your second question, I'm not too sure. It's possible, but from what I've heard, it seems that A LOT of guys book surgery in December, particularly guys in college because they get their winter break and can have surgery and recover in that time. Plus, as from what I know, many guys get it done around that time so that can be more healed during the summer to show off their chest. Since it's 4 months out, I'd say it's worth a shot to book then, but you should do that ASAP.
Hopefully the guys who have actually had top surgery with him can chime in and correct me if I'm wrong, but what a lovely Christmas gift that would be if you could get surgery then!