In regards to food and drinking, you should talk to his speech therapist and OT. He may have an underlying issue with chewing or textures. They should be able to help you.

When we first started potty training my daughter we took her potty every half hour and kept her on the potty for 5 to 10 minutes and gave her a tablet so she wouldn't fight it and it worked about 3 weeks we was in pull ups but has regressed since then. As for eating I'd try yogurt tunes, chicken nuggets and Mac n cheese. My daughter would mainly only eat that for the longest time but now she eats just about everything we do

Yes he is already in these therapy's he already goes to physical therapy, occupational therapy and speech therapy too