HI Shelly Oliva,
Can you tell me what breed this is?
How are the eating and drinking habits? Is he a fast eater, slow eater?
What about elimination, normal? (urine and stool)
Thank you for that information. This breed is prone to hip dysplasia and this may be what is going on with Zeus. Hip dysplasia would cause a stiffness in the joints which would have him shift his weight from side to side. Has your vet checked for this?
They are also prone to patellar luxation which is a disorder of the kneecap. You may see limping with this. http://www.petplace.com/dogs/medial-patella-luxation-in-dogs/page1.aspx
Eating too fast may be a cause of stomach upset. Try feeding smaller meals throughout the day, and putting in an unopened can of vegetables in the middle of the dis. This will cause him to eat slower. There are bowls that have cylinders already in them for this reason. If you find that slowing down the eating helps then you can invest in one of the bowls. Some people will also use a bunt pan.
I am going to direct you to a site on your breed so you may see the prone illness and the treatments. Scroll to the bottom of the page. When you see an illness you want to read about click on it and another page will open.
There is an expert with us that raises Westies so if my information is not helpful then I can direct the question to her when she is online.