I think you should just flat out tell him. I know you think its mean... but really, its meaner to keep him in the dark and keep living like this being miserable and hes probably miserable too. Sometimes its better to just get it out. Then start doing it together. Walk or talk to him while hes on the elliptical machine or something. You are physically not attracted to him, but do you enjoy being with him? His personality and sense of humor? That is a form of intimacy... so talk while hes on the elliptical .... maybe you'll gain back some intimacy that will increase your sex drive while hes working to lose weight.
Explain that you worry about his health too, especially with a child on the way.
Good Luck!