Hi Rose. I too have been working with a 3 year old with an eating problem. His mum did tell me that he had been violently sick when he was just 9 months old, and he progressivley got worse. Now he is 3 he will only eat one or two particular foods.
I tapped on him and showed his mum how to do it too. (I actually tapped on her while I was there because she became very emotional while we were discussing this problem.) I suggested that she tap on herself as she is feeling anxious, and I thought that her son might be picking this up from her somehow.
Anyway, to cut a long story short, his mum phoned me (after 5 weeks of me initially seeing him) and said that her son, after being really naughty, lay down in bed and began tapping his karate chop point and said, "Even though I need to eat other food..." Apparently there had been no mention of eft or tapping for weeks, but his mum had noticed that he had begun to tolerate food being next to him on the table. (He would have normally thrown it across the room!) She had also noticed that he had become more loving.
He wanted me to go back to see him, which I did recently. As of yet, I haven't heard any more. But something seems to be working! It seems to be baby steps at the moment, but that is better than nothing.
I would definitely suggest you tap on yourself for your son. I would use, "My name is _____, and even though I don't want to eat, I'm still a fantastic kid." I would also suggest that you tap on yourself, using the affirmation, "Even though I am worried about _____, I deeply and completely accept myself." etc...
I hope this helps a little. I'm still quite new to eft myself, but I have had some amazing success with it. Keep tapping!!