息子さんは、なかなかの二枚目ですね。
In kabuki, the name of the actor who acts the role of a lover (male) is/was always written on the second panel, from there, the word 二枚目（にまいめ）means:
- the role of a lover, a lover’s part
- a handsome male actor who acts a lover’s role
- a good looking man
So the sentence means: Your son is (rather) good-looking.
To which many older mothers in Japan will say:
いいえ、そんなことは ありませんよ。
No, that’s not true.
but I think
まあ、ありがとうございます。
Oh, thank you.
should be appropriate. Or I will definitely say so!
By the way, 三枚目 is an actor who plays a comic role. In the Kansai area, being funny is a highly valued characteristics but 息子さんは、なかなかの三枚目ですねis not a very good compliment.
