My son was doing this a lot this year, off and on. It was not OCD or a Tic. It was under his breath per sa. I was concerned, because then they can get acid erosion from this constant burping/belching. I keep looking at the diet issues too. One thing is I cut down on sugary juices, as he seemed to be drinking a lot of it. So when he does get juice its cut 50- 50 with water. And I suspect it could have been yeast or parasites. I have him on Alinia and it maybe the one thing that has helped the belching. Thats the trouble when your doing so many things, you don't know what stopped the issue.
So those are 3 things to consider, is it diet, yeast or Parasites.
He does have some OCD, and maybe that is part of it, but the core issue really looks like a GI issue of some type. I say this because (sorry) it sounds like bubbles coming from the stomach and is odoress (sorry again).
As far as asking him, well he is minimally verbal with asd, so that is out. We do talk to him and ask him questions, but that is after he starts belching.
So, it is tought to figure things out when you can't communicate with him. That's why I am trying to see if others have dealt with this sx. Thanks for your insight. John L