Is he in a Prison or a County Jail rozied? When my ex was re-jailed on his probation violation, he ended up in the county jail for the first couple months, and the only thing those guys are allowed to wear is the slippers. Then when he was moved to the prison to serve out the rest of his sentence, they put him to work in the kitchen. He earned his own money. They even had used clothing, left behind by other prisoners who were released, where they could get used stuff for free. The one time I took my son to visit him, he borrowed street clothes from one of the other prisoners so that he didn't have to wear his prison pants.
So please let us know when you find out the answer about the sneakers. I'm curious how it varies from state to state. (And it does.) And just know that if you send him $$$, it won't necessarily get used for sneakers... you can't control what he spends it on.
I think your response to your son is JUST AWESOME and SO APPROPRIATE. You're a good mom. You;re a good person. Don't forget that. Don't forget that it is OK to have boundaries about how you will allow people to treat you and how you will respond when they treat you poorly.
I only hope that when my son is older, I have the strength to stand up to his demands and set limits. It's hard enough when they are 3 years old.