Hi QCA1,
A lot of good advice so far. Outnfree's point about your grandson feeling that maybe controlling his eating is about the only thing he can control is a good one. This would not likely be a conscious decision, but since it is possibly a subconscious one, is evidence that HE FEELS abused or hurt, emotionally.
Also, Bryan's point about him asking for your help... is potentially his last cry.... he may go into a shell and become introverted if you don't help. (Not to put too much pressure on you... :) )
Happy guy's advice is probably the best, once you take into consideration that it would certainly seem that he DOES NEED and WANT help. I think the school approach would be best.
I have four kids and I am going through a divorce. My kids are younger..... 10,7,5,4.... but as their ages go up, they are showing more signs of cries for help, because of mostly emotional abuse by their mother and grandmother, ironically. I am very close to getting them out of there, but because of their ages, I have to do it 100% through the court system. Their cries for help, however, are not dissimilar to your grandson's.
Not to sound disrespectful to your son, but he doesn't know the harm he is causing his own son right now. He is evidently too wrapped in the obey Jehovah at ALL costs mentality, or is potentially being too controlled by his wife (I can relate to him on that one... doh!) and is not seeing the big picture. You are probably best equipped to look out for your grandson's best interest, but if your laws are similar to here in Ontario, Canada, as a grandparent you have no custodial rights to interfere. So it will have to be your grandson's wishes.
One other point that I thought about expressing..... I was never very good at pretend games, so I don't think I could pull it off myself, but if it could work for you, try to see if you can go so far as to let your son believe you went too far and that he was right..... to get back in, on the good side of them and work on your grandson from there. Tell your grandson flat out that you want to make sure he leaves safely, since his dad might just be going through a rough time and not seeing how he is hurting him. Assure him that you are not taking him away for good.... just until dad "feels" better. Even though he hates what his dad and mom are doing to him, he will still need assurance that his paternal bonds will not be severed for good.
I wish you the best,
Brad