Becca, I don't have an answer either. It sounds like your understanding of the protocol is much the same as mine, and I agree with Neil that you should ask his medical team directly. That is the only way you're going to find out.
My husband also went immediately to ATG without any sibling testing, but that's because he's over 50. It's different for kids, and the only reason I bring this up at all is because we also decided we wanted to know if he had a match, and we had to really push for HLA testing. If your insurance won't pay for it, you may have to pay for it yourself. It's not cheap, but with only one sibling to test, it could be worse. It might be worth it just for your own peace of mind. If you find out his brother is not a match, it will be much easier to accept what has already been done and where he is now.
If you find out he IS a match, then that opens the door for the next conversation. If the HLA testing is done by a transplant center, their team will likely review his case and make their recommendations one way or the other. My tendency at this point would be to wait until he does start to relapse, but I can certainly understand why you would want to put this behind you while he's still relatively young. You really need to get the opinion of someone trained in the issues of pediatric AA.
-Lisa, husband Ken age 60 dx SAA 7/04, dx hypo MDS 1/06 w/finding of trisomy 8; 2 ATGs, partial remission, still using cyclosporine