Lets assume, for just a moment, that it is jelousy. The medication Jim and Sterling are taking (Amitripylin) might work. And you might not be able to wean him off of it (Although I have tried, they are better behaved ON the medication). Supposedly, as the vet says, it calms their stress to the point where "They dont care HOW many brothers and sisters they have". (those are her words....LOL)

So, if he gets a clean bill of health, you may still be able to control his "problem".