I feel you, but I have to say that your options are limited.
Theoretically, one of the success keys in a relationship is honesty and communication. Considering this, you should probably talk to him and try together to find a common accepted solution… I know this is hard... I am not an alien. But this is the right thing to do, cause in a relationship you don't take decisions by yourself, but you decide with your partner.
Someone would respond: Ok, let him talk to his partner, but what could they do about it? What difference it would make? Well, as I said in the start, your options are limited.
One option would be to continue as friends, another option to change the relationship type to an open relationship. Terrible choices if you ask me, because I don't think it's easy if you love someone and share your life with him to end the relationship.. But there are no miracle solutions, is it possible that he would earn some weight again?
Anyway, I wish you the best, and sorry for not being of much help.