Don't be worried just yet... Let it settle in for a few days.. Don't handle him. Pick a day next week and pick up a rat smaller or the same size as the widest part of his body. Try feeding him again and hopefully he will take it. If not.. Don't handle him just yet.. Wait another week and try again. If still nothing I'd try an African soft fur (I heard they are like the fillet mignon to snakes) if he still doesn't eat I would just not handle him until he does.. I had a snake go 8 months without eating..

If he is extremely under weight, then assist feeding might be the next step, but that is the absolute last resort. I would weigh him and put a picture up so we can see just how malnourished he is.