My dd has sensory issues, which have reduced a lot as she’s got older. She used to be massively anxious about going to school and it got to a stage in yr1 that she would only wear one washed out very soft blouse and was very close to school refusal. I agree with others saying to wash the clothes. I would start washing them now so that they’ve been through a few times and softened.
I used to whirlwind dress dd at the very last minute so she didn’t have to think about what she was doing / wearing. This reduced the stress around going to school. But I appreciate that won’t work with many children as they need routine and transition time.
If your ds won’t wear the uniform, I’d have a few back up white soft tops he will wear. Dd didn’t always wear polos and had a lot of soft thermal vests from M&S. Even if he doesn’t want to wear the shirt, he may still wear the jumper and trousers as long as they’re elasticated and comfortable.
I did struggle to get dd to school on time and a couple of times she flat refused to get dressed. On those days, I sent her in with her school clothes in a bag and the teacher / TA chatted with her about fitting in. She also wore a headscarf as a security blanket every day from year 1 onwards when she was struggling to go to school. She alternated between not allowing me to brush her hair to being an absolute tyrant over it and wanting a perfect style. I discussed all this with the teachers and they made a sticker chart for her and she got a sticker for getting ready without fuss and getting to school on time. Teachers became concerned that she’d start to get singled out by about year 3 with her scarf and chatted with her. She did stop wearing it. But perhaps your ds could take some kind of security blanket with him?
I also think you should consider deferring. When your child is older, you really will wonder why you were so adamant tbh. There wasn’t an option to defer when dd was little. She’s 13 now. She was ready for school developmentally. But the change and restrictions surrounding were difficult for her to handle. She was the only I was the only child, who cried and wanted me at drop off throughout reception and into yr1.
Above all, don’t worry too much about clothes. The few parents that discussed this with a understood. One had a dd, who refused to wear knickers and took them off as soon as she turned her back and managed a couple of times to go in bare bummed. Another refused to wear socks. The school have seen this all before. But it is monumentally shit that they expect littlies to wear a shirt and tie. So if your ds won’t wear these items, I wouldn’t be too concerned just yet.