The only way a torn cruciate ligament gets fixed is to have surgery.
If he has hip dysplasia, then he may need meds. If it is sever enough, he may need to have surgery.
If he isn't treated, no matter what the problem, it will affect his other leg and could result in more skeletal problems.
Personally, I would of course, get him to a specialist. And keep him lean. If he is too heavy, this will put a lot of strain on his body.
I agree with animalcraker, ask a bazillion questions!
Good luck with him, let us know what the vet says! I hope for him and you it's just a sprained muscle!