Hello again, and thanks for your additional and helpful information.
From your description of his symptoms, I agree that it doesn't sound like bloat, thought some of his actions can be similar to those of bloat, i.e., not being able to find a comfortable place for himself to lay down, and pacing/panting.
If he has an appetite, that's good, and I'd feed him if he's hungry, now. Hopefully, he will keep it down.
It's possible that a combination of the long car ride, visit with your family, their dog, eating a little bit of the other dog's food, may have caused him to feel anxious and/or have a little bit of a stomachache. Try to settle him down by feeding him and comforting him, surrounding him with his 'familiar' toys, blanket, etc. The following will help you identify reasons for excessive panting in dogs:
If he continues with the behavior you're observing, I'd have him checked out by the ER vet tonight, on the odd chance that he may have ingested something he shouldn't have, while on the trip, or he just may feel nauseous, perhaps from carsickness, or anxiety. Bloat may still be a possibility, so if he doesn't start settling down soon, I would consider a vet visit.
I hope Howie is back to himself, soon, and would you please let me know how he's doing and what the vet tells you, if you bring him in? Thanks!