Aspen is safe for substrate, but not for feeding. It can cause impaction. Some bags will even warn about feeding on aspen. 4 of my snakes are fed in a seperate feeding container because I use aspen. The two that won't eat in another container are fed on a "tray". Before feeding them I cover the aspen with a piece of cardboard for them to eat on. I am very sorry about your snake. I too recomend taking him to a vet to find out the cause for the sake of your other snakes.