We adopted our gorgeous 2 year old springer spaniel 10 days ago. He is settling well, but his poo is diabolical. He was with a fosterer for 1 month and she had him on Chappie with a dollop of yoghurt 3x per day as he is underweight. His poos were soft but firm enough to pick up.
Since we have had him, I have been introducing Fish 4 Dogs working salmon and potato so he is now getting 100g of fish4dogs, mixed with a generous spoonful of Chappie, slippery elm powder and a blob of yoghurt. However, since he has arrived, he has had diarrhoea and it doesn't seem to be improving. His coat is very dry and lacklustre although he has bags of springer energy - particularly if there is a ball in the vicinity! 
I know that a change of environment can cause tummy upset, but how long should I leave it before I try something else/ see our vet? Does his diet seem reasonable? He is a short, thickset sort of working springer and I do wonder whether I am feeding him too much.
Any help/advice greatly appreciated!