Have you spoken to the SENCo about transition? For Y6s with ASD we do things like additional visits e.g. an evening tour of the school when it's quiet, a chance to meet the form teacher, a chance to shadow a Y7 (usually with similar SEN) for a day- as well as the transition everyone does.
If possible, have a plan with him for "if things go wrong"- e.g. if he's late out of lessons and misses the bus etc, where should he go? This will likely be his form tutor or SENCo.
Is there a quiet/sensory space he can use at break/lunch if needed? If sensory issues are a big thing, ask if he can leave lessons a little early to avoid (some of) the crush in the corridors and a fast pass for the lunch queue. If uniform is likely to be an issue, speak to the SENCo too.
I would try to speak to his form tutor early on- if they are good, then they can support issues with other teachers and be an advocate. Sometimes the SENCo is too busy.
In my secondary school, if he had more significant challenges, he would spend Y7 in a "nurture group". This group have all their lessons together in a smaller class, with a TA as well (usually the same TA all day who can help them get from lesson to lesson). This helps with transition too.
He won't be the first student with ASD at the school (I know all students with ASD are different) and even if he has significant challenges, he won't be the first student with significant challenges at the school (unless it is very new). The vast majority of secondary schools will really want to support and make transition a success.