Jack Russell Depressed

My Jack Russell is very depressed and listless. I got him from a dogs home 8 months ago ane we bonded very quickly but for the past few weeks he seems uninterested in anything and seems very wary of me. He has always curled up next to me but now he has found a spot at the other side of the room and just lies there.
I know he was mistreated in his last home but has has only TLC since joining our family.
I took him to the vet last Thursday and he could find nothing wrong except a bad tooth which he said was not urgent. Please help, I am so worried about him. If anyone goes near him, he yelps even though they have not touched him at all. The vet did a thourough examination and could find nothing physically wrong with him. I am ill myself with terminal cancer and I am just wondering if there is any way he could have picke up on this fact?