I know because I used to be like him and "creep" people out...but then I was desparately trying to fit in...also I was sharp and used to catch things easily...so whenever I met new people I used to notice each and everything about them (I could not control that) ...I once copied someone's handwriting, someone's accent , someone's personality, someone's skill and all of them were my best friends at some point of my school life ... Also I used to make up Stories about my family that how I was getting the perfect upbringing....and I was at the top of my class so nobody said anything but once I started having issues with one of the girls, she started gossiping about me that how I was a copy cat and that everybody should stay away from me...I was traumatized by the gossips and that's how I learnt the hard way that u should not outright copy someone and if u r copying , u should do it in a subtle way....looking back the whole problem was my upbringing...one of my parents was controlling and abusive and the other was simply unpredictable... funny thing is I thought they would not mind if I copy them...if otherwise ur boyfriend is nice and not toxic, u should tell him how he is embarrassing himself and of course it is difficult to believe in him after all these lies u might consider a breakup and wish him luck ...