Really need help mums

[irishmissie]

m really hoping I can get some advise. My son turned 4 on Tuesday and he is still not potty trained! Firstly let me explain a little about him. His speech was delayed and he is under speech and language and on his last assement they said he was averaging at two years to two an a half. He can talk however he can't say full sentences, so we have "cory come play" Lines like this.... He can however count to 100 and count backwards and spell several words and also recite the alphabet and backwards, knows all his colours but couldn't say mum can I have a drink? His speech is very delayed and he has a niece a few months older than him and one who is 18mths and I would say my niece of 18mths is more advanced speech wise than my son.
His older brother is autistic he has aspergers and while I have mentioned and do believe he is showing autistic tendancies his brother was potty trained and dry at night by 3 n half!!
He has so far done one pee in the pot his was when he was 2 and he cried when I said well done, he will say bye bye mummy when he needs to poo and if I put him on pot he won't do it. We have done cold turkey with nothing, we got an egg timer that was 20 mins and put him on the pot for three days every twenty mins and nothing. On the third day he had an accident first thing then nothing and after lunch I could see he needed to go but he was screaming and crying as he was sore but for love nor money could I get a pee in the pot.

The health visitor and nursery are all involved, but he's due to start school in September and I really don't want him in nappies at school. Please be assured this isn't me being lazy far from it both my husband and I work and we both have tried everything we can think off. If we ask him where do you do your pees he will answer in the pot however I do not for a second think he has grasped the understanding of it he can just remember the words he's supposed to say if that makes sense. Please any advise from anyone in the same boat or success stories