Hi there,
This is not uncommon, even without the current situation.
Firstly do you know about your Local Offer for your borough? If you don't, this is like a big directory that holds all the information/services for SEN in your area. Simply google your borough name and then SEN Local Offer.
My local council offered classes after diagnosis. These were really helpful despite having already raised an older asd child, always something to learn and meet new people. I would ask CAHMS (if that is who has diagnosed your son) if they will be running any classes when allowed? Mine have also run one on sensory issues which covered eating/diet etc.
We have had a referral to a dietician who was quite honest in admitting there probably wasn't a lot she could do in terms of getting him to change his diet. However, she did go through the foods he did eat and identified those nutrients he would be lacking so we could buy the relevant supplements, rather than a generic all rounder. She did give us some information to read, but I found more useful advice from other parents.
I second the advice to look on the National Autistic Society, loads of fantastic information and help in directing you especially when newly diagnosed.
I would still email the school and let them know he has a diagnosis. The staff will still be working (I have had lots of support from mine). There may be parts of the report that would be helpful for school to support him, especially once he does go back. Does he have an EHCP?
When you have a moment to look at your council Local Offer, have a look for the Short Breaks section. There should be lots of advice to get you started.
It's a lot to take on board, it's easy to feel overwhelmed, but you are not alone. 