As everyone else has stated, you need to take him to a vet. Gas will not cause localized lumps like that. There are several reasons lumps like this will form, and each reason is a reason to see a vet: parasites, malignant tumors, and cysts all create lumps like this.
The good news here is that this is not, generally speaking, a dire emergency. If your work schedule will not allow you to take him to the vet within the next week or so, he should still be ok (assuming he continues to act normally). Just get him in as soon as you are able. And I would personally feed him smaller sized prey until the vet makes a diagnosis so that his system is not overloaded in the meantime. If your work schedule is going to be a problem for longer than that, however, I would try to see if someone else, a friend or relative, can take him in for you.
Good luck and keep us posted!
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