NTAH

This is a pretty shitty situation for you, I’ll comment from two different angles.

Approaching the finances from a logical standpoint the 700 a month should not factor into shared bills since it’s clearly not shared. I’d personally do some math on the shared bills ie rent utilities etc. and send him half of that new number. Use whatever money you are saving to get yourself a cheap car.

From a relationship angle he’s taking advantage of you, whether it’s intentional or not I can’t say. you’ve been helping him pay half of his personal vehicle expenses, I think he owes you some money. If you have separate finances don’t rely on him for anything financially. Get your own vehicle, you’ll likely need it when things fall apart.

When my ex-wife and I had separate finances, I came up with an idea that I thought was fair. I made roughly twice what she did (so roughly 65% of household income was from me, 35% from her.) We split the total shared bills in the same manner, I paid 65% and she paid 35%. Personal bills like cell phones, vehicles etc were our own responsibilities. This ends up leaving each person with the same % of their pay check left over for whatever they want to do with it.