I’ve always had issues with gaining weight. For most of my life I was 5’10"(178cm) and 135lb(61kg). The only thing that ever made a meaningful difference(as an adult) for me was protein powder. I could drink nothing but beer for every meal and not gain a pound, but whey protein powder in conjunction with weight lifting helped me put on about 15-20lbs(7-9kg).

I think 10 is a little young to be seriously worried about forcing weight gain though, especially if he’s already got a calorically full and balanced diet.

Quickly looking over their history I’d be willing to bet they are US based and although I’d be happy if that were possible it most certainly is not. An injunction of that nature to censure “religious” speech, even on the slim chance it would be granted, is basically unenforcable here. They would get a slap on the wrist a few times and then maybe with enough evidence a less favorable custody percentage, but by that time a lot of the damage will have already have been done to the child. They’re at a pretty impressionable age.

From the wording of the post and depending on where they’re located, this is a child that is a part of a legal custody arrangement. Removing the step-parent from the child’s life is not a simple thing that is within the power of OP unless the other parent can be convinced which seems unlikely.

They would need to go through the legal system and demonstrate that the child is in a clear and present danger in order to amend their current custody agreement. Ideological differences are not going to be enough for a court to agree to any changes except in the most extreme of cases. At least in the US.