If your talking about pinkie mice those are too small. Food items should be about the same size as the thickest point on the snake.. fuzzy mice are too small too.
Right now you need to try to feed him a very small Petco mouse, pick the smallest one you can and it needs to be LIVE.
Don't try to feed frozen right now, you need to get him eating before you even consider feeding f/t.
A 20 gallon cage is much too large. You need to move him immediately to a smaller cage. That is the reason my ball wasn't eating for so long and he ate when I moved him to his 15q tub.