I would start by checking back with the vet to be sure that the pneumonia has not returned. It's common with these dogs and it will come and go and many dogs succumb to it because the owners just don't expect it to return.
I also think that you should talk with the vet about using a feeding tube to get some nutrition into him.
I would also get him started on a vitamin supplement that contains essential nutrients. Your vet can give them to you in treat form.
I hope this helps more!
Let me know if you still want to discuss anything.