Yes, unfortunately, it is legal to suspend for up to 10 days per school year. in my humble opinion, it's utterly ridiculous to suspend a 5 y/o. I would expect it to have zero impact on him or his behavior - developmentally, he's just not mature enough to get it. Really silly move on SD's part, in my humble opinion.
If you want to fight it, a couple of things come to mind immediately. Was this sub a certified Special Education teacher - in other words, did she have any business being in a Special Education classroom or was she just a body filling an empty space? Was his IEP being followed? Does he have a behavior management plan (BMP) and was *it* being followed?
Personally, I wouldn't waste my energy on fighting the suspension. I absolutely would request an IEP mtg. If your son already has a BMP, it needs to be revised. The specifics of the incident need to be reviewed and a plan needs to be documented so that this hopefully can be prevented in the future. What was the trigger? New teacher? Change in routine? etc.
If he doesn't have a BMP, you need to request (certified letter) a functional behavioral analysis (FBA) so that a BMP can be developed by the IEP team. Remember that you are a member of the team and that you probably know your son best in terms of triggers and ways to effectively manage his behaviors.
Just my lay opinion - I'm no expert.
So very sorry you're going through this.