Hello I Have A 11 Year Old Wheaton Terrier That

Hello I have a 11 year old wheaton terrier that has developed some recent issues. He is a very well condition pet and gets regular exercise of 2-4 mile walks several times a week. Lately his appetite has decreased water consumption has fallen off and has had bowel movemnts of small bile laced discharge. Yesterday he vomited a brownish colored deposit and he is quite listless. He does share time with a younger wheaton, and a 1 year old German Shepard which is having intestinal enzyme supplements. Is there some direction of reaearch information I might be able to source on his condition? Thanks Chuck F.