First and foremost I am replying to your post because I feel so bad for you and your son. I have never been a parent so cannot fully understand how troubled you must be.
If I could though ask a personal question ? then explain why.
Have you suffered from depression? I ask this because my understanding is with animals and if we have an anxious mother then she will usually have anxious offspring. Just pursuing this line if there is nothing you can do about him going back to his dad you could try to pretend that you are not worried so much about the s****e he is feeding him.
No you cannot change how you feel but you may inadvertently be adding to your sons worries. I know this is harsh but if he loves his Dad then any criticism of his Dad would be upsetting for your son.
I would not suggest that you try to ignore the bad food issue just lighten it up a bit with added comments like, good job you' have good features, and so the zits wont matter (ha ha)
I do work with troubled youngsters and the one thing that is the same for all of then is that they may criticise their family but we must never actually join in, merely acquiesce.
good luck and I hope your son soon feels better about his life.
On the issue of animals for research "The question is not, 'Can they reason?' nor, 'Can they talk?' but rather, 'Can they suffer?'" Jeremy Bentham