Immediate relative categories (which your parents will be) do not allow derivatives. So they will come and get their green cards, and then they can petition for him. It will be about 2 years if he still is under 21 and about 7 years if he is over by then. Both faster than you filing directly for him (sibling visas are 15 years +)
Note as long as your parents are just LPRs and not citizens, he will need to remain unmarried to be sponsored by them.