If you go by the rule of "a prey item should be between 10%-15% of the animals weight", that is a tad heavy. Using this method, a rat 9g-13.5g would be accurate. This isn't always an exact science though.
If the rat is much bigger/wider than that thickest part of your snake, I'd say hold off for another week or two (keep feeding the 7g rats for two more weeks). If the 15g and your snake are about the same size in width, or your snake is bigger, you'll be fine.
He'll be fine with the 15g rat. This is the time that their appetite and metabolism allows for decent sized, more frequent meals. As long as he doesn't end up looking like a speed bump sign for longer than a couple of days, there's no harm in upping prey size quicker than the math tells you to.
Fully agree with Eric. When I got my smallest he was 153g and I immediately started 19g fuzzies he's now eclipsed 200 in the month I've had him and he's on 26g pups! The math is a good rule of thumb but I use the size chart of biggest part of body even if it means jumping ahead a bit!