How To Toilet Train An Adult With Autism?

Hi my son is a 37 year old autistic. He is non verbal and also intellectual disability. He is quite physically healthy. In recent months he is urinating less frequently. He comes home from residential care every 3 weeks 3 weeks ago he urinated around 8am. His next urination was around 5 pm. Then he went a third time around 11 pm before bedtime. But today he got up at 6am and I stood him at the toilet . He did what I thought was about 70per cent. The flow seemed to be a bit weak . He then did not go for the rest of the day even tho I tried him at the toilet numerous times- he was quite resistant to being toileted. I left him back to his care community at 9pm and he still had not performed . I ask them to ensure he was toileted. I d like to know what you think P J Lambe