only son
Refers to a male child who is the sole son in a family; may be used when there are daughters but no other sons, or when the child is the only one overall.
彼は一人息子で、兄弟がいません。
He is an only son and has no siblings.
一人息子を甘やかしすぎる親もいる。
Some parents spoil their only son too much.
He was raised carefully as the only son.
His parents dote on their only son.
My father wants his only son to take over the family business.