Unexplained Crying Advice Needed

Hi.. my son is 5 with a diagnosis of ASD and is non verbal.

He often will cry at bedtime for no apparent reason and due to being non verbal he can’t tell us why.

Tonight the crying session has lasted way longer than usual.. nearly 2 hours. He is sleeping soundly now but I find it so distressing. 

I worry maybe there is something wrong that I’m missing because he can’t tell us how he is feeling. 

I have mentioned him doing this to his consultant before and he said it could be a sensory issue, he certainly didn’t seem concerned by it. He said it could be he finds feeling tired upsetting. 

I was wondering if anyone else’s child does this. It may help me not to worry so much to know it is common for children with Autism to do this and any tips on how I can console him would be greatly appreciated as nothing we have tried has helped up to now.