This is not about you. This is not about your son. This is about his behavior and he is the one who has to look at himself in the mirror each day. You are co-parenting with him. You are doing it. You are co-parenting because it is in the best interest of your son to see positive parenting behavior and you are an example to your son.
Don't guess and second guess his actions. That will only deplete your energy. Whether or not he is "doing this to upset me" is not an issue. His motivation is not something for you to evaluate.
Be careful WHEN you read the emails. Skip over them when you are not ready to read them. Or, simply print them out for posterity and get to them when you are in a good state of mind. You do not need to respond to him immediately.
Keep your eye on the ball. With every action, ask yourself, is this in my son's best interest? Are handing over the complete receipts to your ex in your son's best interest? Yep. Getting help from his family? Not your business nor does it in any way affect your son.
I'm saying this to help organize how your look at the situation because this can be so overwhelming!
I hope your son feels better soon because on top of all of this a sick child can be a bummer in itself:(
Best to you!
Jen