I"m sorry to hear of this with your 15 year old, Gillian. This type of cramping can represent a simple partial seizure, primary muscle disease in that hind leg, a peripheral neuropathy, or a myelopathy (spinal cord disorder). How often are you witnessing this? This would present quite a challenge in determining the etiology and then treating effectively. I might entertain having him on a prescription nonsteroidal antiinflammatory drug (NSAID) to see how it affected him.
Thank you for your kind accept. I appreciate it. We're necessarily constrained by how much we can give a 15 year old but I would first see how meloxicam (an NSAID) affected him and if it weren't helpful see how adding gabapentin (a neurogenic pain reliever) to the meloxicam affected him. Skeletal muscle relaxing drugs such as methocarbamol might be of value. Feel free to share our conversation with his vet.