We’ve adopted a 12 year old border terrier with little history. We’ve been through the gobbling food so he made himself sick saga, now we’ve got the runny poo and seems to be straining when trying to go saga. We’ve checked all the initial things, food, strange objects, dehydration, illness, stress etc. Nothing is answering our questions. Do older dogs do this??? Learning from scratch really. Any opinions will be gratefully received. Thank you.