Formal/affectionate:
* ליבעלעך קינד (Libelekh kind): This literally translates to "little darling child" and is a very affectionate term. It's often used for young children or someone you feel a strong connection with.
Informal/endearing:
* מאָינס שוועסער (Moins shveser): This translates to "my sweetie" and is a more informal and playful way to say "sweet boy."
* שניידעלע (Shnaidele): This is a diminutive term for a boy and implies "little sweetheart" or "cute boy."
* שײַנער (Shaynor): This means "handsome" and can be used to describe a boy you find attractive.
Other options:
* זיסער (Zisser): This means "sweet" and can be used to describe someone who is kind or has a good nature.
* גוטער (Guter): This means "good" and can be used to describe someone who is well-behaved or virtuous.
The best way to say "sweet boy" in Yiddish depends on the context and your relationship with the person you're addressing.