I don't think wedding rings should be compulsory in a marriage, however if he normally wears one, I don't think he should remove it. That's what cheaters do. If he wants someone who is willing to join you, he needs them to know about you. Plus, I would feel terrible if I started a relationship with someone who had a partner and didn't tell me, even if the partner was fine with it. At that point, it's cheating on the new partner.
It does make it hard to find people, but you want someone who will be fine with the situation anyways, so having them be warned upfront is probably a good idea.