Yes! My son is the same age. No EHCP though - current bone of contention without not fit for purpose SENCO.
Firstly, there is no way in the world I would be able to get my son to wear his uniform if it wasn't a school day. Similarly, after one amount meltdown during "world book day" I have sworn to never buy into non uniform day nonsense again.
Apart from this being a possibility. Is there anything else that might be bothering him? Like the seams or the texture or the washing labels or have they been washed yet? Do they smell like his? (your washing powder).
Can you put a favourite top of his under the jumper? Or if they are navy trousers, have u got some alternative navy Joggers he can wear?
Can you do a social story or a preparation story?
Reception are so lenient with this sort of stuff. I'm sure he won't be the only kid in the class who is resisting the conformity that uniform brings.
If all else fails, take it from me, don't force the issue. Send him in with the uniform in a bag. He might get changed once he is there and realises it's the norm because all the other kids are wearing it.
Also, think about the first day/week. The kids will be so nervous or excited they won't be paying any attention to clothes. Kids that age don't hold the same concerns about fashion as we do.
And the parents will be too busy crying at the gate and taking photos of their own kids to worry about who is wearing what.
Plenty of kids in my son's class (he was in school nursery where uniform was required) rocked up in all sorts over the past year. A boy came in full power ranger fancy dress, a girl used to come wearing pink leggings giving zero cares, another
boy wore white trainers, my son once when in with his sunglasses on, a wooly hat, carrying an umbrella, wearing shorts and a winter coat. He couldn't have cared less what people thought. I cared. But all I got was banter about dressing for all weather because we never knew whether it was going to rain, snow or shine from one minute to the next at that point. I think that day was the day I made most of my mum friends due to having a talking point.
Good luck xxx