Thanks so much for the informative response. I never thought there could be a tongue tie because I assumed it would be visible, the frenulum under his tongue looks normal to me but obviously I'm uninformed if you've got to feel for it.
He wets his nappy pretty much every time he feeds so I haven't been worried about that but I have no idea if the amount is little or a lot.
Our post natal classes are run by a specialist nurse who's also a lactation consultant and my GP nurse has signed me up to have some sessions with her or her colleague, I'm sure they'd be informed on tongue ties. I have a class tomorrow so I'll mention it. She weighs baby for me every week because she has an analogue scale and everyone else who weighs him uses digital. The GP nurse and pediatrician also weigh him. Nobody has suggested anything that could be causing the issues apart from my milk just not being "enough".
The teet doesn't collapse because we use anticolic teets that have a little valve on them but he does get extremely gassy every day anyway. You're right about why I prefer the slower flow and we already do paced feeding with him. His sucking reflex is really strong but I don't really know how else to tell how much he actually needs other than offering more.
He had jaundice and lost loads of weight at birth, he has a dilated kidney that needs constant monitoring and he takes a low dose antibiotic to prevent UTI's, he has sepsis and only maintained his weight while on antibiotics in hospital for two weeks, he lost weight during the following week at home. So yeah, he's been through a lot.
I'm incredibly sleep deprived and baby is very colicky, I'm really struggling in a lot of ways, I barely look after myself, I've continued to wake him for feeds at night because of his weight but I think I won't tonight because I need to sleep, hopefully his weight is good tomorrow.