It doesn't sound like the food to me. I think right now it's good that you have cut him back some but in my experience it takes a while for ferals to figure out that they have a permenant food supply. Plus he is young and will eat a lot anyway. I don't think you have done anything wrong at all and it sounds like it could be the dreaded coccidia! He will be fine once he gets some meds and maybe a diet for a while. Sometimes it can be a booger to get rid of!
Other than that it sure sounds great! Maybe Zelda likes him more than she is willing to admit! They will have to work out the pecking order and I'm quit sure Zelda will immediately let him know that she is boss! Once that is all worked out I think they may become friends! Orange boys somehow have a way of making that happen!
Keep us posted on his belly.