Good Lord, OP, You are a moron. He is already emotionally fragile and you have to really stick it to him. Congratulations, he might very well commit suicide. Just mind your own business. Honestly, people like you do more harm than good everywhere you go.

I had a cousin in a similar situation. Everyone knew that his "mom", who treated him poorly, was really his stepmom. I am fifteen years younger than he is and even I know. IT came out when a drunk uncle told him that his mom wasn't his bio mom at a family party.

So I won't tell you what to do, but maybe there is something to be said for controlling how the info gets to him.

Maybe the grandfather is really the father and the Dad is really a half brother. And the Dad has always been mad that his Dad slept with his girlfriend the Mom and he had to raise his Father's son

Maybe the mom lied to the dad about the baby being someone elses because she was really mad at him.

Maybe the dad lied to his family that the boy wasn't his because he resented the boy.

Maybe the dad told this to his family to explain why he isn't having a relationship with his children and it's not true.

gossip, rumor, dishonest granny, who knows?

Let brother know what grandma said so you aren't lying to him and offer to test to see if half or whole siblings.