Poms Behavior

I have a three year old male pom and for three days he has been acting under the weather, meaning he doesn't bother much with the other poms and cocker spaniel i have. He lays on the living room floor alone and he will eat but not much.

I was sitting another dog for a week. He got along with the dog but didn't bother too much with it. The dog went home on Sunday and ever since then that is when his behavior started. What can i do? His health is good but he was born with one testicle.