I would first go with whatever doctor said. IV users can get abcesses from shooting drugs that can appear that way. However, you didn't say what your son's issues/choice of drug is, so this may be completely irrelevant.
The fact that he doesn't live with you anymore makes me wonder why you would want to drug test him. Is there some kind of other arrangement you have with him, ie helping with his rent or other things that would matter if you drug tested him.
I know how it is to worry about your son when there are signs for concern, but there isn't really much you can do about him. What does he say? Most important is that you don't let it completely preoccupy your thoughts and time , especially if all seems relatively well otherwise.