JRT sick?

by Jennie
(Tifton ga)

My 3 year old JRT is behaving different. He used tone very hyper, ran, jumped and barked often. He is an outside dog, but comes in daily to play and eat. We used to have two JRTs but they started fighting so my husband and I decided to rehome one. Since we haven't had the second one as long as Alex, we let him go two weeks ago. I've noticed a change in Alex since letting mickey go....he doesn't run for food, doesn't jump on the kids as much and doesn't bark as much. This past weekend we went out of town. My friend came to the house to feed Alex. She called me to let me know Alex was making a funny noise and acting too calm. (Not barking, not really eating, not jumping, laid down like he was feeling well. I noticed the difference when I got home. Took him to the vet, she said, heart and lungs sound good. Temp good, teeth, gums, and nose good. No heart worms. She thinks he misses mickey and us....Can that be true? What else can I do? Should I take him back for X-rays and blood work?