I found fedup.com.au helpful, has info on mums who've experienced noticeable reduction in symptoms of ADHD, autism,tantrums, bedwetting, rashes, hyperactivity and more with reduction of food additives.
1st you could try only using additive free(282, 281,etc) bread and preservative free milk (need to research on internet which brands as I only know ones in Australia) I think there's a site whatsinyourbasket.com too?
U could stop buying nuggets/chips (its OK he will be so much better off, though he WILL complain, which is hard for you). There is probably flavour enhancer and/or MSG (620-625, 631,635, yeast extract 637,HVP etc) in the nuggets which causes hyperactivity and natural yellow(annato,180b) in the chips which causes headbanging/tantrums in many children.
Keep presenting him with additive free food, though expect him to refuse, just keep calm, let him refuse, try again next meal/snack time with another additive free meal and his hunger will eventually ensure he does not starve- he will be OK and feel so much better in time/though will protest. Need to allow 2 weeks-a month to see changes in his food preferences and you should see a great improvement in his symptoms too.
Main additives to avoid nasty side effects are:
colours102,104,110,122,123,124,127,129,132,133,142,143,151,155,160b
preservatives200-203,210-219,220-228,249-252, 280-283 in bread
synthetic antioxidants310-12, 319-321, BHA,BHT
flavour enhancers 620-625,631,635,yeastextract,HVP
flavours-avoid strong