Having read a few of the responses, it does seem that, although the boy is clearly difficult for the parents to manage, they do have a duty to ensure he stays quiet during the restricted hours at the very least. Autistic or not, there can be some effort made on their part. It may not produce perfect results but you need to assert your rights as tenants too.

I would start with the note but invite a face-to-face conversation. I say that as a parent who was asked by a neighbour to keep my son's activity down during the lockdown (and he only leapt around for no more than 2 or 3 hours each day). It was communicated to me via email and was quite curt but I also think that written notes can be misread and can seem cold so inviting a compassionate conversation may feel less confrontational and cold. Explain that you understand it may be really tough on them but that having these noises stretching into the night or the early morning as well as all day is too much. You do need to clarify your basic rights too, just as they and their son do.

If that doesn't resolve things within a reasonable amount of time, then I would take it further.