My eldest refused to breastfeed at all while awake from 10 weeks. :( I had to dreamfeed her during every nap and throughout the night, which was exhausting and horrendous to the point that I became very depressed and anxious about it, but somehow we got through. She had a posterior tongue tie finally diagnosed at 4.5 months and had that cut. I assume your son has been assessed for posterior tie if he has seen a specialist? They're a little different to conventional ties in that you can't see them, only feel them via palpation of the underside of the tongue, or the use of a grooved tool to pop the tie out of hiding. They can cause all sorts of problems and because not many professionals are clued up on them in the same way they are with grade 1 or 2 ties, they often go undiagnosed, leading to lifelong problems with food and diet in some people. Since they can also appear in conjunction with more traditional ties, practitioners who are unaware of the existence/implications of posterior ties will not cut far back enough to fully solve the problem.
Just a thought. Unless you're absolutely sure posterior tie was looked for and ruled out I'd definitely give that another shot. If you're anywhere near Cardiff I can recommend a brilliant practitioner who will be able to tell you once and for all.
Cutting my daughter's tie did help - she finally started to feed awake from about 6 months, though we had to do it lying down while she learnt to cope with her new mouth and my rather aggressive flow. By 10 months everything was much better, though she took a very long time to chew solid food properly, going waaaaay past the age she should have started doing it (I had a SALT come to the house to assess her in the end, who determined she was ok, just on the later side and gave lots of helpful tips). However, she didn't stop choking frequently on water/start drinking out of an open cup until she was almost 5! That was a huge source of anxiety for me, but again, we got there in the end.
I still don't really know for sure what caused it all, but as well as the tongue tie, I think she may have had some problems with her tonsils and adenoids, as she is prone to rhinitis and eczema. With age, though, both are improving.
She's 7 now - no lasting difficulties with food or drink, speech impressive, development completely normal.
I can see you're really anxious about this and I would be too. I, personally, would stop worrying about solids too much for now, as he is still young, and start trying to get as much milk into him as I could via dreamfeeding if at all possible while working on getting to the bottom of this. You could start by getting the tongue checked again if necessary, then if it's not that, maybe you could see a SALT in case it's low oral tone or something in that ball park which is causing these problems. You say he struggled to keep a hold on the breast - this could be caused by both tongue tie or issues with oral tone. You also say he won't take more than a little milk - again, if he is getting easily exhausted as he doesn't have the oral power to maintain a grip on his bottle for the duration of a feed, this could imply he needs some help strengthening his mouth muscles.
Again, just a thought. Our problems were not as bad in the grand scheme of things as this sounds for you but still I felt utterly devastated over it, so I can only imagine how you must be struggling right now and I wish you the best of luck.