Hello, sorry OP I just came back to this.
I agree with Sparkly entirely. It is good to manage our children's diets and our own well, but that's not relevant to this issue. The more you focus on the dietary aspects and controlling it that way, the worse it may become as he will feel that he is being forgotten with all the fuss about his eating.
To answer you question Hokey, I didn't overeat as a child. I underate. I basically stopped trusting that I could eat and feel alright. I survived on very tiny amounts of food and my parents were understandably scared and they concentrated on this more than on what I needed which was basically masses and masses of reassurance, all the time. I needed them to sit down with me and say 'what are you feeling, love, and how can we help' but they didnt - they tried to breeze me through it by being 'normal' as possible, but also kind of 'come on, dont be ridiculous' sort of stuff which though I can imagine why they were exasperated, didnt help.
I know they didnt understand but I really needed to be asked, to be held and allowed to cry and be a baby and be scared, and not have that dismissed as 'silly' or 'it will never happen, just get on with it or you'll starve' or anything that tried to get me to be top-down normal again.
I definitely sensed their fear which is why a counsellor might have been better...however, if they had taken me seriously - said Ok, we will help you with this, whatever you need to do' it would have meant that there was someone there who accepted me, as I felt, and was there on my side completely. So, in terms of what I could eat, they had no control over that at all - whatever they did I was scared of food.
But with the emotional support in place, I could have found my trust and security returning much quicker. In the end I found coping strategies. But it took a long time and these fell apart as things continued to feel out of control at home, and school, and in terms of feeling safe. I felt alone, that's the best way I can put it. Maybe your son feels abandoned by his dad, maybe scared of losing everyone he loves in the same way, it'll be irrational but very powerful fears.
CBT is good with children. My Mum got a few techniques from a book and tried those with me. Just her attention and her not screaming at me to eat really helped more than I can say.
I do hope this helps a bit. Just love him, listen to him, don't try and make him do anything. Work with him on the eating by all means but it's a side issue. Ask him how he feels, what he is afraid of and accept those fears are real to him. That can be very valuable to a kid in this sort of situation.
Good luck with it, to both of you x