Youre a lovely mum becasue you clearly care and want the best for your child.
Portion sizes - we've all been trained by the supermarkets and restaurants and fast food industry to have bigger portions than we need..
My dd is a skinny and i worried she wasn't eating enough! Turned out it was me that had it wrong, I was (still am
) overweight myself and joined was my a few years ago (for me nothing to do with dd) the first thing I learnt was my portion assessment was way out of whack! I was serving myself 2.5 times what an adult portion should have been AND expecting dd to eat half that, which she could never finish so I thought she wasn't eating enough. Soon as I adjusted that we were both happier.
I see he doesn't like meat/fish does he like chicken? I'm veggie so there's plenty veggie protein, soya, beans, pulses, nuts (don't have to be whole nuts, nut butters/cutlets/roasts/pâtés), quorn. Cheese n eggs fine but cheese especially high fat plus you don't want him getting bored. Protein makes you feel fuller quicker too.
Instead of custard creams what about rice or oatcakes? Or half a crumpet? Or muffin (English not American cakey ones). Or malt loaf or half a teacake? Or breadsticks? My dd loved garlic/herb ones around this age.
Dried apricots also high in protein and yummy.
Where I live they just started a healthy cooking scheme aimed at parents who missed out on cooking and nutrition classes at school and didn't have healthy example at home. It's being marketed along lines of 'not sure what's healthy with all the confusing messages? Want to learn some new recipes?' And seems popular, perhaps there's something similar in your area? I heard from a friend who's been they're talking about healthy oiks for cooking different recipes etc too. Perhaps there's something similar in your area? Your HV would know.