I'm no lamb expert, this is only the second year of having any here and last year they were all on their mums, but with shaun we broke his feed down into small but often so when the others were on just 2 feeds, shaun was on 4 smaller ones (he was really swelling like a puffa fish)
he also had a short course of fast acting antibiotics but the odds were stacked against the poor little thing (never had colostrum it seems) and we had him PTS. Not saying that Larry has the same sad fate but these bottle lambs often have sad and difficult starts in life
You are doing your very best and its a real shame that the vet is not calling back. I'd change now, get some urgent advise from another practise to put your mind at ease if you can x