take your chances if you think he's fine.. everyone here says to take him to a vet but i dont think they realize that a vet cant just put a cast on him and send you off.. so basically your gonna go there and pay him $150 for him to tell you that he has broken ribs but theres nothing he can do!
i would first monitor his breathing, if he's making funny breathing noises then the rock could of damaged his breathing tubes.. also you might want to feed him smaller portions now.. if he's in pain then you dont want to make it worse with a HUGE rat.. so you might have to backtrack his feeding cycle..
if he still seems really messed up after a week or so then you might want to get him checked out but really theres not much they can do for broken bones!
hope this helps