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Hello. Thanks for the question.
The hind leg spasms can be due to a couple things- neurologic disease, joint disease/ arthritis, a toxin or just general pain. My guess here would be arthritis and pain. This is very common in older dogs. We can see trembling, pacing, panting and restlessness.
I would talk with your vet about starting an anti-inflammatory medication like Rimadyl. Sometimes we'll also add an opiate (Tramadol) or something for nerve pain (Gabapentin). If you are unable to get him seen and you just want to try something at home, you could try Aspirin. The dose in dogs is 5 mg per lb body weight. It can be given every 12-24 hours, as needed.
I hope this helps, let me know if you have any other questions.