First of all, if you purchased that puppy you need to immediately check out your legal options for puppy lemon laws and things. If your dog is already having health problems at 7 weeks old that is a sign of very bad breeding. If he was adopted, there's really not much you can do unless the SPCA or anything has a written health guarantee (unlikely). You should see about taking out pet health insurance ASAP as well, because one problem is just going to lead to another.
I'm not sure that a limp would be related to a murmur.. I see what the vet is saying but I've not personally ever heard of that. Heart problems can easily make the dog tired and inactive. It's important to keep an eye on their energy levels- too little is not good, but too much can lead to problems as well.
I am not a vet, but from what information you have posted I wouldn't say there is a good prognosis in the long run.. Mastiffs are already predisposed to problems, and although sometimes heart murmurs can go away, the limp and suspected genetics issues lead me to believe your pup may not have quite the normal life expectancy
My first dog was a rottweiler from a very bad BYB, and she had to be put down at 7 after having genetic issues from day 1. Hopefully you will be able to work with your vet to resolve or prevent some of the issues