Ex chef here.
I’d really REALLY recommend your son keeps it short and simple, at least till he has some experience under his belt. Does he have catering experience? Sorry I can’t remember if you said
There are two issues - managing a long list of ingredients and ensuring you have everything in stock. That is not easy - it’s time consuming and also there’s a cost implication for holding large amounts of stock. Does he have dedicated fridge and freezer space? Ie not within your domestic fridges at home? He’ll need a lot of space and environmental health will not want things keep with your own home “stuff”.
There’s also the pressure and logistics of delivering a wide range of cooked items on time, piping hot and perfectly cooked. The simpler the menu, the less difficult that is to do.
Pizzas are a good bet IMO - nearly everyone loves pizza, the ingredient cost is relatively low, but it won’t be easy to keep them hot during delivery if you’re in a rural area. They are also tricky because if you have an order for say 5 to one address, can he cook that many at the same time? I know our local pub get into terrible difficulty with that very problem - they couldn’t churn them out fast enough if more than say two people on a table ordered them.
I’d avoid like the plague any high risk items (I mean from a food poisoning perspective) like chicken wings, and I’d also swerve ice cream (good luck trying to keep that cold and your pizzas hot). The cakes idea would be fine as long as you stipulated say, order minimum 24 hours in advance & no refunds.
Finally in terms of pricing very roughly you’d be looking at allowing 1/3 for cost of ingredients; 1/3 for overheads including delivery, and 1/3 for your son. In my experience many small offs businesses the owner ends up working for minimum wage - I know I did on the food side of my business which to the outside looked like an enormous success.
Food is notoriously difficult to make money from although it’s incredibly satisfying when you get wonderful feedback! I wish you all the best of luck