If he cruises and walks holding your hands, I'm not sure what your doc would be looking for with the x-rays except possibly hip dysplasia? But he should have been checking for that starting as a newborn... and anyway, your son probably would have trouble cruising if he had that problem.
I suggest that if your son isn't walking in a month you ask some detailed questions about what the doc is looking for with the x-rays, what diagnostic alternatives there are, what the risk to your son is (as far as dose of radiation), whether waiting another month would be harmful in any way, etc. Is there an early-intervention physical therapist that might help? This will help you decide whether there is a true need to do the x-rays, should they be necessary. Remember, you don't HAVE to consent to anything - just get the information so you can make a good decision.
Yes, some kids don't walk until 18 months, but 15 months is the late side of normal. You're right though - some kids are just so happy with crawling that they don't have much of a drive to walk. If you think this is the case with your guy, just do what you can to encourage him - he'll figure it out soon enough.