Snake digesting mice poorly

I have a snake who will be 20 in August (I bought him when he was 3 months old). In all this time he's been healthy with no problems. He unfortunately eats live mice because 20 years ago I couldn't buy frozen mice and when they did become available I couldn't get him to switch over but I've never had issues with his feeding before. Six days ago I fed him his standard 2 live mice which he killed and ate with no problem. Two days ago he either passed them through partially digested or regurgitated them, I can't tell which. The smell was horendous and what was left of the mice was gelatinous and disgusting. Now he's acting hungry again and when I put my arm in the cage to clean out the mess, he tried to bite me (fortunately I was anticipating it by his movements and was wearing thick sleeves). The only vet in the area charges $90 just for a basic exam so I don't want to take him just to hear that he's old or something. If anyone has any advice, I would greatly appreciate it.