There Are No Bad Precious Ones in the World - Chapter 19

Easy, this kid was an only child and since he has a new sibling (also the baby is a boy) he believes the baby is taking away the parents love.
Solution, spend a separate bonding time for the 1st child.
Say that when he was an infant he was treated with the same love and affection and will continue to do so as they grow.
Explain that when their sibling grows up they could play games together, that he could teach his little brother things he knows.