Is my DGS just a late developer or could he be autistic?

DGS1 is almost certainly on the spectrum His aunt was diagnosed in her twenties, and he has presented almost identically from a very young age. Nursery gently raised the question with his parents. A friend who is a child psycholigist agrees. He is in a highly supportive private school environment, so as yet there is no drive to seek formal diagnosis. He benefits from having a range of coping strategies which were very helpful to his aunt are also now working for him.

My point is that he is in Year 1 and - again, like his aunt - already reads well and avidly (shop signs, road signs, childrens magazines etc). Maths is at a similar level.
He has been wiping his own bottom since he started Reception, aged 4.

There is absolutely no correlation between these skills and autism!
Please tell your friend to do some decent research before she offers you any more ridiculous speculations, OP.

Many children (and adults) on the autistic spectrum may be in advance of their contemporaries with regard to reading and also for maths, etc. It is the social skills that they lack.
My eldest g.son, definitely autistic, was actually given a 'Talented & Gifted label in his first couple of years at Infant school. However, he found things like sitting in one place, lining up, playing any sort of team game, dressing up, virtually impossible. He took instructions very literally, so being told by the teacher to sit on the hall floor (for assembly) he thought meant he had to sit on ALL of that floor. Sorted by giving him a carpet square and told to sit on that, etc. With help of Teaching Assistant, he got through primary school, but it all fell apart very quickly at Secondary school, even one where there was a reasonably good SENCO department.
My daughter and SiL fought so hard, getting their own private reports/assessments to get him into a Special School for Autistic youngsters. Yes, it was horrifically expensive, which is why the LA fought against it, but just over three years there, he came out with a whole raft of management techniques for his condition as well as excellent GCSE results. For sixth form he has returned to LA College and has now passed Maths A level with a B, takes Physics next summer and Chemistry the following summer. That expensive few years has given him the means of becoming a useful member of society for the rest of his life, and thus a much cheaper option in the long run.

Just going to say the same thing. Getting an autistic diagnosis is hard to do. It must be done by a psychologist and not just because a child is deemed to be immature.