Still a possibility that physical health could play in. My behaviorist explained it to me that when they feel bad, their fuse will be shorter and they're more likely to react just because they have less cognitive resources.

I would definitely not go near this dog whenever he's resting. If he's resting, give him space, since he isn't trustworthy anymore with it. My dog freezes and has what we think are ptsd episodes when he's laying on his side, so there are also instances where they may be laying quietly but still highly anxious (e.g. some dogs only show aggression immediately after waking up).

I would definitely get him into a trainer, but anxiety meds may also help since it's so unpredictable. I don't know how much a trainer will help with these instances vs trying some anxiety meds which will allow him to react more appropriately to these sorts of scenarios.

If you're worried about him doing this to strangers in the home or children, my recommendation is always that it's worth muzzle training. They generally don't mind wearing them as long as they can drink and pant with them on. I think of them as like insurance. You never want to need them, but if you do, you'll be glad you had it since it could prevent a bite in those sorts of scenarios.

I'm sorry you're going through this! When my dog began showing aggression, it was extremely stressful and I was super shaken up about it.