I'm not sure what you mean. By the rules of Chess960 he can castle from either b1 or g1 (b8/g8 for black), and can do so either queenside or kingside.
If you're saying you can't do that while playing online, just move the king onto appropriate rook.
If you're saying he shouldn't be able to castle in that position, I disagree. Allowing castling is what separates Chess960 from Shuffle Chess, and keeps it similar to Chess while voiding opening memorization.
If the king can't move over two no castling (like when king is on knight 1).