He doesn't sound constipated to me, so don't worry about a problem you don't have. He's just not that hungry. You don't need to step down his prey size...
just give him a few more days to digest it. You probably need to offer food (of the current size) about every 10 days, no big deal.

Constipation in snakes can be the result of poor hydration (not drinking enough water & humidity too low), so you can offer f/t that are thawed in water &
not dried (if he'll take them that way) or inject water into them. To help a snake that is constipated can be a soak or preferably a swim in the bathtub, as
that combines water and motion. (totally supervised of course) Snakes don't usually get constipated, or if they do, it can also be the result of having formed
marble-sized urate stones (also called "cloacaliths") that literally get stuck blocking their cloaca, and once those are removed, you wanna duck. You'll
probably know if your snake has a urate stone...you can gently feel around the area just above their cloaca...they feel like a marble has lodged there. Best
to see a vet for help with this issue.