10-y-o male schnauzer doesn't eat as often as he used to

by Valentina
(California)

First important question I need to ask, is it possible for mini schnauzers to get that problem where their stomach flips?

My dog loves to eat and sometimes never got full and always wanted to eat more. We always gave him scrapes of leftovers like ham, chicken and beef. He played with really small bones that sometimes I felt were too small.

So he turned 10 old in June and I been worried about him getting old and sick. A few weeks ago, he wasn't feeling well because he had a bloated stomach. Now he sometimes doesn't eat the regular dog food we give him (first thought he is tired of it) he did barf that food a few times these past 5 weeks even though he has eaten that food for maybe 5 months and so has our chihuahua who hasn't had any problems.
When he goes for a walk, he poops very little to nothing. like about the amount or less of chihuahuas. He is urinating well and drinks lots of water.
Do you know what might be wrong with him?