I think sometimes kids can get a little jealous of all the attention we give our Yorkies.  My daughters gave up and just joke that I'm "replacing" each of them as they move out the of house.  They say I'll need to get a boy Yorkie as soon as their little brother grows up (he's 14) and moves out. Okay. 
hmm, only one of my Yorkies is "sassy" - not a bad trade! lol 
However, it would be advised to talk further with your son and see if it's jealousy or simply him being unable to relate, let him know that yes, you love you dog but it's still a dog and is second to him.     
				__________________    Kendra     |