Hello John,I am so sorry to hear your little rescue Shih Tzu is not himself at the moment, the poor munchkin... It’s very distressing when something is wrong with your little darling and you aren’t sure what it is… I would suggest taking him down to your local vet as soon as you can for a more in depth discussion about him and a thorough physical examination. It would be particularly important to discuss whether it is that he is struggling to urinate or defecate, when you talk about him having problems going to the toilet. Both of these issues have many causes and may require prompt medical attention. It may also be the cause of the changes in his behaviour and appetite that you have noticed. However, without more information and good physical exam of your little mate I am unable to give you any detailed advice. It is possible there could be multiple things occurring so definitely head on down to your trusty vet for some help.
I hope you and your vet can get to the bottom for what is going on! Let us know how you go.
Best wishes and Merry Christmas!