M.G.
If it were me, I'd be returning the sweatshirt.
My kids are not allowed to make purchases of large amount without our input at that age. Even our older kids have to get our weigh in before buying anything of large amount.
I don't know how it works with parents who are not together. Here, you aren't allowed to go to one parent over the other. For example, you can't ask mom if it's ok, and get a 'no' then go and ask dad. That's the case with anything.
Sounds like you have two issues. One with your son not understanding the value of money. Two - it sounds like you have to come up with a better way of parenting your son with regards to how to make decisions where mom and dad need to be involved. What are you going to do for example in a couple of years when he wants to buy a car, etc?
I get it - I'd be upset too and don't really know what your ex was thinking - I think I'd cool off and then deal with both issues separately.