A few things:
I batch cooked for me and DP for PP because I have had anorexia and I know food won't be my priority
If you’re a family of 3 (and currently you are) and you cook a meal, you cook for 3. Maybe your DSS wants it, maybe he doesn’t. But you cook for him regardless. Batch cooking is the same. He might be there and he might want some, so he needs to have the option. He’s not some second class citizen who has to eat something else.
I am more comfortable making him separate meals with less salt and spice etc
As you will see when your own child grows as well (although hopefully beforehand with your DSS) - generally it’s easier to I just eat all together and reduce the amount of salt/spice you use.
I was not asked to make any meals for him as DP has fresh food for him
But if he often gets chicken nuggets as it’s too late this system doesn’t work
He finishes nursery at 5pm and goes sleep at 7pm, I don't think he should be having big meals just before bed he needs time for stuff like that to go down
THIS IS WRONG. Please do NOT follow this advice. Same with him having had a hot meal at nursery. In the nicest way, it sounds like your own food issues may be clouding your judgment. Children need several (ideally) warm and filling meals a day. Doesn’t matter if it’s just before bed!