Tongue tied baby breast feeding

I’m really struggling with my 5 week old son who has a tongue tie, we are waiting for his procedure.

In the meantime he is formula fed, i top up with breast sometimes which he really seems to want.

But when he’s feeding on the breast he gets very upset, coming off the boob very often and aggressively going back, pulling the nipples, getting upset with the boob in his mouth, wiggling. I don’t know what to do, when I squeeze the boob I can see milk coming so I am not sure why he is getting so upset, can anyone help please?