My Tosas have had their fair share of tumbles, and have had a limp here and there. I watch them over the space of a week, to see if it's getting worse. Luckily I've never had to take either of them to a vet as they've gotten better within the week. Definately keep him as still as possible, short leash walks only, until you know more what's going on. if you're lucky he'll improve...