It's just the phrasing "lose fat" is the problem...Try to look at the positives!
For example: if he starts going to the gym and putting on muscle, any more fat he incidentally gains will be less of.a burden on his mobility and he'll be able to do fun things with you without getting out of breath or hurting his back. Plus, the strenuous exercise will increase his appetite.
I'm a pretty gung-ho feeder, and I'm lucky to have met a feedee who's the love of my life. But as much as I love fattening him and he loves getting stuffed, I still insist that he accompany me in my exercise so that we don't get to a point where his mobility and quality of life are compromised. Of course, I would love him and stay with him if he decided to get so big that he needed help in and out of the shower and whatnot, but we've had the conversation and decided that that's not the life for us for now.