AITA for wanting to use my boyfriend and I's joint account to help my mom with a bill?

My boyfriend and I make money together. At the moment, we're saving for a trip and all of our earnings go into a joint account. We pay for everything out of that account and rarely use individual accounts. We have a different view when it comes to finances; I believe you should still treat yourself with something, whether it be a pedicure, a new blouse or even a trip to the cinema. He, on the other hand is a religious money hoarder and will live like a hermit in the name of saving money for said trip.

I had a job up until September that was very damaging to my mental and physical health. Everyday, he would beg me to quit it and go into business with him full-time. Eventually, I did and I told him that if I leave that job, all of my pre-existing debts and future expenses would have to come out of our joint account, to which he agreed to.

Over a period of 3 years, I've probably spent about $1000 on personal items for myself, whereas he's spent about $200. However, that's because he will live like a homeless person with stretched out boxers that are falling off his ass to call himself a "saver". He also has this thing where if I buy something, he has to buy something of equal value. For example, if I buy an iPhone because that's what I genuinely want, he also has to get one too because I can't have anything that's worth more than his. The thing is, he may genuinely want a cheaper phone but he still would take the full $800 because we have to be equal and I can never have more than him.

Anyways, I still live at home and recently, my mom brought up that I help out with the Cable bill which would be $100. I don't pay any other bills, so this was a pretty reasonable request to me. I told him about it and he freaked out and he doesn't think that I should contribute to my mom's bills with money out of money that he owns too. I got upset and told him that I'd go back to making money aside of him because he literally polices every purchase that I make. He lives with his father and even though his dad doesn't require him to pay bills, he would still take $100 because I'm helping my mom out. If I knew that he was going to act this way when it comes to money, I wouldn't have even quit my job. I know you may say all of this can be rectified if we allot allowances to each other, but because of this trip, it's kind of difficult, so we just take from one big pool. Plus, he doesn't want me to make money aside for myself.

Am I the asshole for spending money from our joint account to help my mom out with the bill? I understand where he's coming from because he believes in equality and he has nothing to do with my mom but at the same time, it is a JOINT account. Also, why would he force himself to spend the same amount as me if he genuinely doesn't need/want to?

TL;DR - I want to help my mom with a bill from our joint account and boyfriend thinks it's unfair to him.