My DS is in year 2 scoring 1a-1c and he has a statement, full-time support and needs an individualised curriculum as he can't do the same work as the other children because he has autism and severe language disorder. You should absolutely win with scores at P7 in year 4. Govt guidance says a child really needs to be at level NC3 to go to mainstream secondary. It is pretty unlikely your child is going to go from p7 to NC3 in 2 years in his current placement. He is going to need specialist education. He needs it now.
Its not just about lack of progress - its about rate of progress compared to peers, whether gap is getting wider, whether progress is in line with intellectual ability (do they even know what his is?) and whether support needs to be above what a school can offer. Its also about non academic targets - social, emotional, life skills - look at SEN Code of Practice for egs
If he doesn't have full-time 1:1 then who is helping him do the work? What work is he actually being given to do? He must be getting completely separate work to the rest of the class so what support does he have to do it? I would ask to go and observe and see what he is actually doing. Senco saying they are doing all they can / you won't win is perfect excuse - you can say I'd like to come in and observe some lessons so I can reassure myself. But really you will just be there to collect evidence of what happens when he doesn't have support.
Look at IPSEA refusal to assess pack for ideas of how you need to present your case. Charities like contact a family, IPSEA, NAS can provide a caseworker to help you with the tribunal.
You say school don't know what to do. So they need more input or he needs to be somewhere else. Does school know what good progress for him looks like? Do they know how to teach in a way he learns? Is he capable of learning more?
DS went from p3/4 to 1c/b within weeks last year when we realised school were not teaching him properly and we (and private therapists) stepped in and did it instead. It turned out school were hugely underestimating his abilities. The teacher spent no time with him herself at all and didn't know him. Just left him to 1:1 to teach. There was no plan to fill his gaps or weaknesses he was just given the same work as everyone else and the 1:1 left to try and adjust it so he could do some of it. We've now made school provide an individualised curriculum. But we had to prove to the school they were underestimating him. I have to be on their back all the time making them do it. It was very convenient for school to decide nothing they did would make any difference as he couldn't learn anyway - it let them think it was ok to do nothing for him! If he had gone from p3 last year to p5 this year they would be telling me that was good progress for him whereas he's now working at 1a. So what did they know.
If you wanted your child to go to special school then I think you would have a good case. Its not just about now you need to think ahead. Year 7 is not that far away. Year 6 is going to be all about getting as many SATs passes at level 4 and 5 as possible. I doubt your child will even be able to be taught with the group at all that year. The child I know from last year in that situation was out the room with 1:1 most of the year.
I would suggest you go and talk to special schools or units etc and look at them and find out what level the children in their classes are working at and see whether he would fit into any of those. Finding the right people who do know how to teach him can be a huge relief even if its not initially what you wanted for him.