I think you know that you have 2 options.
1) Your milk supply is driven by demand. So as long as you continue to supplement, your supply will only go down over time. If you want to continue nursing, you need to stop supplementing. If you want to do this, you could try drinking mother's milk tea to increase your supply on the same day you stop supplementing.
A good lactation consultant could also help you here. I noticed in other posts that it takes you a long time to nurse your baby. That makes me suspect that he doesn't have a great latch - which may mean that he's not getting much milk NOT because of your supply, but because of his latch. A consultant can also give you other tips on increasing supply - foods to eat, etc.
2) Switch to formula. If this will make you a better, happier mom, then maybe this is the way to go. Your baby is already getting formula, and if he's not gassy or fussy, then you know he can tolerate it. It's not the end of the world.
I would suggest that you meet with the lactation consultant before you decide. Would you regret it if you didn't at least ask if there was something simple you can fix with his latch before you decide to stop?
What did I do? Nursed both mine until they self-weaned. It wasn't always easy - with my 2nd child in particular I had to make several visits to the lactation consultant early on to get latch issues worked out. But it was worth it to me.