Personal hygiene and cleanliness challenges

My 27 year old son has been recently diagnosed with autism. He also is dyslexic. I specifically would like some advice on personal hygiene and cleanliness challenges he has. He lives with my sister and his 2 male cousins. I see my son every 2 weeks and do fun activities together (movies, sports etc.) We get on very well. My sister recently expressed concerns to me about my son's personal cleanliness and how unclean his bedroom is. Dirty dishes are not returned to the kitchen to be clean. My sister is hugely understanding to my son's condition, having 2 autistic sons herself and she works as a teacher in a school for children with special needs. However my son has not cleaned his room for over 6 months, hides dirty plates etc. and the room started to smell before my sister identified the issue. He has not cleaned his clothes and bedding for 6 months, but rarely had body odour. The fact that he is messy is not the big concern, but the concern is that he does not seem to be concerned about the dirty plates and left over old food in his room, all over the floor and in his bed. This has now caused vermin in my sister's house, who is generally a very house proud person. Once I found out, I helped my son clean his bedroom. It was disgusting, but after helping him clean and having a heart to heart with him, I thought I had got somewhere with him. Now just 2 weeks after doing this deep clean and general tidying up with him, it is back to being full, of dirty plates and opened food cartons strewn all over his bedroom. When asked about whether he has open and old food in his room, he lies about it. He has recently got a full time job at a very large retailer in London Oxford Street, socialises with friends...but seems oblivious to the filth in his bedroom and cleaning himself on a regular basis. Can anyone help me with some advice, direction and whether this is linked to his autism?