You have to make sure that he cannot do that again. You could try to take him out of the tank and get a separate feed tank lined with paper towels to move him into when he eats. If you can't do that...cover all of the substrate with paper towels before feeding him and supervise him closely to make sure he doesn't drag the food off of the paper towels and into the substrate. Aspen can be very dangerous when swallowed. You need to watch him closely over the next few days...make sure he is pooping normally...watch for any signs of distress...a swollen vent, vomiting, etc. I hope everything turns out okay! Keep us posted. I just saw that you ordered a critter keeper...good thinking...I use those for feed tanks too.

Edit: Do you have a exotic or herp vet to take him too if he should run into serious problems?