He is doing that because his true character is coming out. People put their best foot forward when they are trying to impress someone and now that he feels like he's been accepted he is showing his true colors. The person who should put a stop to this is your mother. Shame on her if she doesn't.
In the meantime you should change your username to WorthyChild. The only time that negative words can hurt you is when you accept them as true. Your subconscious hears something being said over and over and after a while it accepts what is repeated as true...BUT you can deliberately say positive words about yourself MORE OFTEN and then that is what your subconscious will believe. This is the most important information that I can possibly give you. Protect your mind with all your heart.
What a loser. Stay away from him. I knew a person with "perfect children." Alas, they're characters made me sick. The youngest male tried to learn an instrument, but he never tried, he'd just run downstairs and cried. They were homeschooled by the way. I was compared a lot to them. I was publicly schooled.
Supposedly, they were brighter and could do more work in less time, having more time in the day, but they bragged about it as though it made them superior. I was informed of the inferiorities of those of publicly schooled. It made me angry. They were condescending.
This man is going to produce adults who are going to have seriously troubled coping skills. They'll turn to drugs and will be arrogant and abusive. Grin and bite the bullet for now and disappear when you can. Try to avoid any contact with this fellow, or just approach him and tell him exactly how you feel. If he returns to you with sorrow, forgive him, but if he returns to you with an excuse, he's ignorant, stay away.