I just want to say OP, you are not alone. He may have additional needs and, as you are his mum, you have possibly observed something that you can’t quite yet articulate.
My own DC took multiple years to fully train - even, gasp, after they went to school - I tried everything, talked to nursery, talked to school, went to the GP, HV, paediatrician and everyone I talked to was utterly mystified. But ultimately there isn’t a lot that anyone else can do about it!
In the end, it just took a long, long time for DC to finally get exactly what it was we were wanting them to do. Even then, they thought that they only had to do it once - not every single day! 😂There did turn out to be some additional needs involved, but they didn’t emerge until later.
Nearly a decade on, they are doing well at school, picking GCSEs and it is a long distant memory.
Please try to be reassured that this will get solved, it will pass and it doesn’t actually reflect your parenting abilities.