Personally I'd wait a week after you got him to feed, he'll be fine not eating for a few extra days and will be even hungrier and more likely to eat.

I personally feed in a seperate container because I don't want to risk substrate consumption, but you can do it in your enclosure it's up to you. The first time I tried to feed in enclosure my idiot boa just got a mouthful of eco earth and missed the mouse entirely, eco earth won't do much damage but it's personal preference and what your snakes will do, some refuse to eat unless it's in their enclosure anyways.