I see 2 options. Continue to only offer foods...and hope he will come around eventually.
Or just give baby food and let the mom do it on her own.
I would pick the route that makes my life easiest as you clearly are not going to change the family for the better without a lot of aggravation on your part.
How disruptive is the kid crying all through nap to you? How miserable is he? If he's stressing you or the other children out with his hungry whining then I'd just give baby food and tell the mom that when she is ready to do her part at home you will do your part, until then he gets the food he is trained to want.
I don't think it's worth your energy and stress to try and fix this problem. Take the easy route and enjoy the peace you get from his belly being full!!