Hi Samantha and welcome to Pet Talk. I'm so very sorry to hear about Jack and it was good to see he was feeling a little better and even brought you his Teddy. That is so heartwarming.

I do think it is possible that he may have had a stroke. My childhood dog had several before she went to the Rainbow Bridge in 1999 at the age of 15. Even though she had a few strokes, she could move around ok, if only a little stiffly. Her head was also cocked to one side, but she still had that precious eskie smile the whole time.
Concerning his food allergies, there is also a possibility that some of the other symptoms could meerly be caused by what he eats. Have you tried any chicken based foods with supplements? My vet swears by diet and says that chicken based foods are the best. There is also the option of a natural diet also. I very good friend of mine feeds a raw chicken, chicken liver diet ground up with supplements and it has done wonders.
Also, since he's a senior dog, some of his stiffness could be age-related. As long as he seems to want to go on, it's worth every chance to help him feel even better. All fingers and paws crossed here and I will also follow this thread to see how Jack is doing.
**hugs**
Karen