Rachel, sorry you are having a difficult time :(
Have you thought it might help to talk to someone in real life about this? Mumsnet has the nos. of all the bf lines and you could call one or more of them.
Can you give some details about your baby's weight?
Tongue tie does not have to appear very 'extreme' to interfere with feeding, so your midwife may be mistaken in thinking it does not need treating.
However, there may be other reasons for the issues arising with your feeding.
Your baby is not feeding very often - has anyone talked to you about what you might expect? 3-4 hourly is really fairly infrequent - most babies of your little son's age would be breastfeeding much, much more often. Frequent feeding (as you prob know) is important to stimulate production as well as to get more milk into the baby.
Keeping your baby close and responding to early feeding cues helps with this - avoids the baby getting upset because he is desperate to feed, and also means you get the 'right moment' to offer as you are aware of his needs quickly.
If there is a concern about weight and intake, then you can also increase supply and intake by making sure you offer at least both breasts each feeding session - 'switch nursing' also helps, which means you go back and forth, switching sides when your baby seems to lose interest each time.
Yes, expressing can be a useful thing to do, but it should be in addition to frequent feeding, not instead of.
If you want to introduce formula alongside breastfeeding, then you need to be aware that it's quite early for your body to maintain a supply if formula becomes a regular thing....any formula undermines breastfeeding, and gaps between feeds reduce production. So if you give a bottle of formula and this means your gap of 3-4 hours between bf extends even more, you will make less milk, and production will dwindle. This is especially likely to happen if breastfeeding already seems not quite enough....an option for you would be to fix the breastfeeding so it is enough, and then see about intro'ing formula, when your supply would be less at risk.
Hope this helps. Do post again with more info.