I have autistic twins, and my DS is nearly 16 years old and still in pull-ups, day and night. So I’m always on the lookout for an indication of SEN on posts like this.
However, from what you’ve said that doesn’t seem to obviously be the case.
If I’ve understood correctly, you’ve not made a concerted effort to potty-train, just tried “a bit”. Kindly OP, that’s not really on. It’s not fair to send a child to school without even trying to potty train them.
You’re also talking about him as if he’s much younger than his peers. None will be 5 already, unless their birthday is in the first week of September. All the children will turn 5 between September and the following August, so if anything, he’s turning 5 quite earlyish.
Do you think you tend to baby him, rather than encouraging him to develop independence?
Presumably he was OK at nursery but is now crying and clingy at school - why? Is it just the change of setting? Or have you potentially said anything to unsettle him “Mummy will miss you so much now you’re at big school” type of thing?
Nursery often don’t identify SEN so the fact he is “sensitive”, doesn’t like writing, still uses a dummy, and isn’t potty trained COULD all be potential indicators that something isn’t quite right. In isolation none are of particular concern but when put together it MIGHT indicate something more.
However, when you have a SEN child, it’s a process of elimination. We have to take positive steps to prove it’s not our parenting before SEN is considered. You need to get him toilet-trained as an urgent starting point, as well as figuring out why he’s suddenly clingy. Does he settle quickly? Does he cry when he sees you at pickup? Is he fretting about going back in the next day? All are important questions to consider if you think there’s a possible delay or SEN. But you really have to consider your parenting too and whether you’ve done what you should to get him to where he needs to be.
If you’re comfortable there’s no SEN, then you need to proactively help him to become independent and adopt a brisk approach to dropping him off, not “extra cuddles” which make separation harder.
It’s very, very early - Reception kids are still very young and it can take a while to settle.
If all else fails and it’s just not working, you can opt to home educate for a while. That would give you the extra year at home and you can put him back in school next year. However you’d have to put in the work to help him develop the necessary skills because otherwise by the time he rejoins school he’ll be even further behind in terms of his social development.