Although medical causes need to be ruled out, it could be a behavioural issue, though his paws will be itchy and sore now causing him to keep licking them even if it is behavioural at its root. I have a dog I rehomed which licks himself sore at times (but only when left alone), but it is behavioural and we have managed to break the cycle for now.

The things I tried, which in combination seem to have worked are:

-'Security blanket' with my scent on it, given to him only when he is alone.

-DAP diffuser

- filled kongs when alone - although they only last a little while, I think they must help build a positive association with being left.

- loads of stimulation and exercise at other times.

- trying to minimise the periods left alone and only gradually increasing.

Whilst I don't know the cause of you dog's licking, these measures might help a bit, although medical causes should be ruled out first.

I know how frustrating this can be, and your dog's condition seems more serious than mine Good luck.