Are you weighing him with a regular scale or a gram scale? I would suggest weighing him with a gram scale, as a regular scale won't be very accurate with a rat. Honestly if he had gained that much weight I would be concerned and take him to the vet. It can't hurt regardless.
Also how did you change his diet? Is his calorie content lowered? Are you free feeding now as opposed to having feeding times?
And just to be safe, even though it's not necessary, you may want to consider getting him x-rayed to make sure his heart is alright. You also may want to give him a blood sugar test to make sure he doesn't have diabetes.
I don't think it's going to kill him right away being overweight, but at this point I have got to think that amount of weight can be harmful to his frame as well as his heart and his overall health.