And... he's not the baby daddy

Many moons ago, my boyfriend and I were on a break. During that time I started a new relationship and became pregnant. The guy became abusive so I split. Guess he was a rebound fling and never looked back.
 
The boyfriend wanted back in the picture and adopted the kid as his own. There are no other kids just the one.  
Should we tell him or take it to the grave?
Kid is 19, now. 
(Random pic from Pinterest)

Vote below to see results!