On the one hand, I remember DS waking hourly at that stage, I'd just gone back to work and it was traumatic for everyone.
But on the other, stopping bf entirely at 6 months seems very sudden. He will have become much more efficient at feeding, so wouldn't be feeding for as long. Your breasts might no longer feel full, but will be more efficient at producing milk, so he may be getting everything he needs without that uncomfortable feeling you get in the early weeks.
They say food is for fun until one. Therefore milk should still make up the bulk of his diet. Formula will be harder to digest than the breast milk he's used to, which might be causing him some discomfort.
He is also used to the reassuring closeness of breast feeding. I would imagine this is a difficult transition in itself.
If I were you (and I'm not, but you've asked for ideas) I would resume bf. it's been a few days but your supply will re-establish (I had a stop and restart at 23 months with no probs).
Feeds will gradually diminish as solids and water increase their significance, but this shouldn't be till around a year. I would imagine he'll continue to wake in the night for a while, but bf should make these swift to deal with at least.
Good luck.