Do you have any friends with safe, small dogs that you could let him play with? Maybe he's in some sort of back pain because the way he arches his neck/head seems a bit unnatural, like he's trying to make his back shorter. I would take the video to the vet and maybe if they aren't willing to figure it out, try emailing the video to another vet you trust. I agree that in the end, it could also be that he senses something is wrong
Just this morning, when I took Stella to the vet for her spay, she must've sensed I was worried because normally she LOVES going to the vet (it's weird, but she loves going and playing with all the techs there) but yet she was crying while we were in the waiting room. I've never heard her cry like that and she had no reason to, so the only thing I can pin it to is that she knew I was upset/worried.