Separation anxiety

[Kated16]

Hi, my 3.8yo ds was diagnosed with asd in December, I just wondered if anyone had any tips or advice for dealing with separation anxiety?
It is particularly bad when I am still in sight but he cannot get to me or has been told to stay where he is for example, leaving him in the car to get petrol, going outside to do the bins or put a wash on (our washing machine is in the garage & no direct access), having a conversation outside of the car when he is inside, he will follow me upstairs even if told I am just popping upstairs to grab something. He is generally fine if I am out and he is looked after by grandparents, he also goes to nursery 3 mornings a week and drop off is usually no issue.

TIA