You could make a guess, put whoever you catch in with him, see how they interact. If they get along well (maybe some minor introductory stuff) leave them together, if the one you add looks to be chasing him mercilessly or being too aggressive, take them out.
It is a good idea to pick birds that you know you won't be keeping, test them with him first. When I got my 3 chocolates at 8 weeks old or so, I picked a very docile silkie from the group I had and let them hang out for a few hours at a time. Although silkies seem very accepting for some reason, I had 0 drama. Even in Dec when I got the new 9, the oldest is a few weeks older than the others and hadn't been in with them before. He was happy as a clam at high tide to have the new friends and never even pecked them. Ah, love silkies