I'm sure your boy is different to mine. You might wish to describe his S&L issues, and the teacher's attempts to address them, in more detail.
DS1 outgrew his issue by the time he started school at 4.9 - they didn't affect his ability to access the curriculum. So it was a true "Delay".
DS2 is still very behind - he's 4.8 but his comprehension is very similar to that of our "typical" neighbour's girl who's 3.6. Fortunately, he doesn't have to start reception till 5.0 (he was due to start at 4.0 but that wasn't feasible). He is on school action plus. The headmistress comes to my nursery "parents evenings" which last half an hour and also involve the school SENCO. So we have a good level of service.
With DS2, staff are very on top of his language in the school nursery. Now that he can answer some (only some) questions, that gives them a useful diagnostic tool to see what he has and has not understood. Staff are also on top of his progress in terms of socialising with his peers, which is good.
Ds2 is a strongly visual learner. Teachers really struggle to understand this (I suspect because most people going into teaching are very strong verbal learners). The SALT comes in to "train" them but unfortunately there is rivalry between the two professions so the teachers don't really "take ownership" of the idea of supporting visual learners - they see it as something to grow out of.
Specialist teaching support - in my case, someone who could draw pictures to help DS2 understand what the teacher was saying - would be my dream. However, before contemplating a move as you are doing, I would ask to be a fly on the wall in the new classroom and to watch the classroom assistant work with the kids who have needs.
Much sympathy, it's a tough thing.