You are between the devil and the deep blue sea, here.
You have all my sympathy.
The decision to have a much loved pet put to sleep is hard.
Horribly hard, and for you it is complicated badly by the fact that your husband does not feel the time has come.
(Mine always left the decision to me, and funked that last visit to the vet too, although he did, as long as he was able, dig graves for the cats. I thought it was hard having to make the arrangements and weep all over the vet's consulting room alone, but I realise now that I could have been in an even worse place).
I suggest you phone the vet and say that the medication is doing no good, and that you feel that the time has come, but your husband still thinks the dog has some quality of life and "that we are not there yet". The vet might be able to suggest different treatment. (Some hope!)
The dog enjoys his walks - is this so, or does he go to please Master? Does the dog enjoy his food, or pick at it? If the answer to these questions is negative, the vet might well concur in your judgement, but would he then be willing to assess the dog and discuss things with your husband?
Only you know if your husband will be relieved if you put your foot down and tell him the dread time has come, or will upbraid you (afterwards) for murdering his dog.
Could you fake an attack of sciatica or "a bad back" so convincingly that any "accidents" are left until DH is home and deals with them? That might change his mind, said she nastily.