If he’s the last person in the clan (which would make him the clan leader) you should be ok, he has to leave his clan first but when he leaves his clan, everything will still belong to him and when he joins your clan it will all carry over to your clan.
Recently my wife had a solo clan with alternate character she left the clan joined with us invited her in her belonging we’re then ours it worked have done it one other time. Ps
It was official server