I'm going to text the son of my ex-fiance.
Why do you want to text his son?
I'm going to tell him that he needs to ask his dad to send me $500.
I do not think that you should be contacting your ex through his child. I think that it is wrong, especially if you are just contacting him so he can ask his dad to send you money.
Tough shit, I think there are medical expenses he should help pay for.
If your ex owes money for past expenses then it's perfectly reasonable for you to expect him to pay them, however it's wrong to contact his son in order to make this happen. The son should not be involved in these issues as they are between adults and asking him to do this could be confusing and upsetting for him.