Oh OP, I had so hoped you and your little one might have had some respite.
With good weight gain and no evidence that the extra formula helped satisfy him, there does seem to be something else going on. My money would be on silent reflux which could be caused by a few things.
It could be dairy intolerance, maybe try dairy free formula for a day while pumping your milk and altering your diet in the meantime. BF will not be affected by this as long as you pump to keep stimulating supply.
A very poor latch I think would result in poor weight gain, so I'm not convinced he's not latching well. But taking too much air in while feeding causes problems. One of the biggest clues of a poor latch on my tongue tied baby was a tiny clicking noise as he sucked. This was his tongue flicking over my nipple instead of it working the breast under the nipple to draw out the milk. I had very sore nipples.
His latch was poor no matter what I did until his tie was cut (twice over three weeks, so it took a while). He was my 3rd BF baby so I had experience by then. Latching is very difficult to direct on, you've kind of got to experience a good latch to know the other was bad, not helpful I know.
A good BF group or one to one support at home would be ideal. I appreciate the last thing you want to do is go out to a group, but it could really help.
Infacol and Gripe Water can also help a little, but these only treat the symptoms not the cause, definitely worth trying though. And a dummy, the sucking helps to pass bottom wind. But also do plenty of burping too, so many different ways to try, you'll find what works for him.
And don't lie him completely flat, you need gravity to help keep his milk in his tummy. Raise the cot/basket with a book or two, or folded towel under mattress.
You will get through this. It's a case of trying lots of things, a process of elimination. Sending love. X