I'm uncomfortable with the idea that if you are fairly sure a child won't thrive in a particular setting later on, then it is best to send him earlier in order to train him up for it so it will come as less of a shock later. Parents are under a lot of pressure when they feel that the clock is ticking, that there is a deadline for everything to be sorted out.
I don't know whether you have a particular need for your son to start school early on, such as needing the childcare so you can work? If not, then why rush? Would it not make more sense to send your little boy to school only when you think it is very likely that he will enjoy it and get on well there, and will have access to the help he needs? That might not be next year, or even the year after. The older he gets, the clearer his needs will become. Some of his current problems may disappear altogether with time, and become non-issues. Those problems which do persist will be easier to assess when he is older.
My dd is nearly seven. As she approached Reception age she was so unready for school that I didn't even seriously consider sending her. She was still in nappies, couldn't communicate well, had the attention span of a flea, and had no interest in letters or in numeracy. Most important, she just wasn't emotionally ready to be away from her family and in a large busy setting without much individual adult attention. I wanted her to have a happy start to her education, and I didn't think that would happen if she started school at four. Now, three years on, she is doing so well with home education that I have no plans to send her to school unless she wants to go. But I feel confident that if I did send her, now would be a far more appropriate time for her to start than when she was four. She still needs a bit of help using the toilet, but she never has accidents and always asks for help. She can talk the hind leg off a donkey. Other problems persist, which are easier to evaluate now, so there's no danger of getting fobbed off. She's coming up for a second assessment with a neuropsychologist, and his report would be hard to disregard when it comes to providing for her needs in a school setting. Likewise, her physical therapist and occupational therapist have useful input. You can access these services (as well as speech therapy) through the NHS for a child who is not at school.
I can, sort of, see the argument that in order to get a child the support he needs, you may have to chuck him into a place which can't meet his needs. It may be that that is necessary in order to demonstrate beyond a shadow of a doubt that he just can't cope there without extra help. Sad, but apparently often true that this is how the system works. But even then, that doesn't have to mean that sooner is better. For a child who isn't very mature, you could do it when the child is six instead of when he is four.
There's something else that bothers me, OP. You say you are worried that your son will be picked on if he joins the class later than the usual time, and someone else suggested that his future classmates may see him as more babyish if he spends next year in preschool instead of Reception. I hope that isn't true, and that they will accept him. It seems to me that young children are not so nasty - the ones I know aren't. But if you are familiar with your son's future school or classmates and you do see such a culture there, then it isn't a good place for your son. It wouldn't be a happy environment for any child, especially for a child who isn't average. If you want your child to learn good social skills, he needs to be with other children who are displaying good social skills so he can learn from them. Don't send him somewhere where you seriously expect him to be excluded for being different.