Senior Schnauzer - mobility issues, "legs splaying out"

Hi everyone, I’m looking for some advice please. I have a dog who has just turned 16 years and 4 months old. His mobility has really gone downhill lately. I took him to see the vet and he was prescribed Cladaxxa, Gabapentin and Hedylon. After a week, there was an improvement – he was standing better and walking. He’s still on the Gabapentin and Hedylon, but I’ve noticed he seems to be getting weaker again, unfortunately. His legs keep splaying out from under him. He’s still eating, but his appetite has dipped too... Has anyone else been through something similar? Did injections help at all? I’m just not sure if there’s anything else that can be done for him medication-wise?