All your kids will know their sib is adopted, right? So it's not a matter of them not understanding there is bio family out there, it's just a matter of explaining he has an extra grandfather.
Many of us have step parents, half sibs, etc etc.
I think it's as normal as you make it.
I struggle with telling about bio sibs (his only sibs probably ever) who want nothing to do with us right now. You are lucky your son will grow up with his sibs!
(I wish I had that problem to be honest!)