Folks this is a very serious problem and is not cute for Gus-he could die for it. Are you aware that the salesperson could have put in a report of a dog bite (it doesn't have to break the skin) to animal control. You might want to read up on the laws of Ca but you could be sued and Gus could be destroyed for x amounts of bites. Just because he is small & cute doesn't mean he is protected from these laws. Go walk the pounds and see how many little dogs are in there for biting children or other people and they are on death row. In Arizona it can be 2 or 3 times and they are put down. The judge doesn't care if he is cute or small. The law is the law.
http://www.dogbitelaw.com/PAGES/civil.htm
I also don't know if you have had Gus around children but you will need to be extremely careful based on his reaction to your cousin he could go after a child the same way.
I have had 3 dogs come into rescue because of their biting and if I did not take them they would die. I have been bitten more than my fair share and been bitten down to the bone because of their early training or lack of socialization. If I take any of these dogs out in public I always have a muzzle on them because if I didn't and they bit one more time they would be dead. For the most part these 3 dogs never go to public places unless absolutely necessary and I never allow anyone to pet them.
It is your responsibility to protect him from himself. Hopefully you will read up on the proper way to change Gus's behavior or you could escalate the problem.
I wish you all the luck in the world but it is going to take some work.