I have now realised that he is usually almost normal by the end of summer holidays and for first half if Autumn term. Then, seemingly like clockwork, his communication, tics and sensory difficulties worsen after October half term.
I taught secondary level for years and the latter half of the autumn term tended to be worse for all kids nt or not. It's generally the longest half term so they are really tired. I think wind and blowing leaves are difficult for people with sensory issues (my ds finds wind the most disturbing of all weather types) as well.
Mine is happier in colder weather, so it's the summer term he finds stressful. There is also the fact that being at school can be stressful in many different ways for our children, like walking an extended tightrope that needs all your concentration. The longer you have to do it, the harder it becomes. By the second half of the Autumn term, the level of stress and exhaustion is cumulative.
could be the change in clocks/darkness, I think it is recognised people are more likely to feel tired once the hour goes back. also the Xmas half-term is long and has lots of events that distract from routine.
2nd half of the autumn term in Primary schools can also be very disrupted in the run-up to Christmas with plays to rehearse, carols to learn etc. My DS certainly had a worse time this half term and we used to keep him off school during the week before Christmas because he couldn't cope at all. The routine gradually breaks down and everyone gets excited.
I think stillsmarting is right: we have found that anything that disrupts the routine (school plays, staff illness) makes things worse. Seasonally, I would say that the worst months are January and February. Maybe because his teacher has run out of patience and ideas by then...