Monitor his movement and if it is swelling as these could be signs of a break if his bones aren't the strongest... If he seems to be favoring other legs I would say a vet trip would be in order... How they get bruises is the problem, if he fell there could be underlying problems that as a prey animal they won't show until it's bad.. So to be safe I would just try to monitor it... I can't remember the name of it but there is a cream some people use to help bruises... Try the search feature for bruises and see what comes up... Hope this helps! Iwish I could help more /:
EDIT: misspoke... If all it is is a bruise, it should fade in a few days.. Just monitor movement