Your problem is somewhat unusual, and you may feel the same about my suggestion. If your son is completely toilet-trained and only rarely has bedtime accidents, I would give him his bath in the evening and dress him for school right then and there—everything but shoes and maybe socks. Make sure you don't dress him in anything so tight it would interfere with circulation, and obviously you wouldn't dress him in his best clothes if they would wrinkle too badly. If he protests, just say in a matter-of-fact way that you are doing this to avoid the morning struggle and to help him get to school on time. Children's clothes aren't so fancy these days (mainly T-shirts and boxer shorts) that he will look rumpled in the morning. If this works, praise him and tell him he can go back to getting dressed in the morning if he prefers. But one fit and it's back to bedtime dressing.
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.