Q:

What does あの子、あんなんだしさ mean?

Would you please tell me what あの子、あんなんだしさ means?
and then she said あの子を理解してあげられる人なんて　たぶんごく僅かだと思うのよ。。。
The person who said this phrase is a mother talking with her son's friend about her son.
Thank you in advance

A:

It means something like

He's always like that

あの子 (or この子 if the child is nearby) is a standard way of referring to your own child in conversation.
(だし)さ is displaying a mild concern (he's always playing with his food, he's always getting his clothes dirty, etc.)
し is actually the listing particle ～し～し, but often used by itself for emphasis in colloquial speech, implying there are many more things she (presumably as mother) is concerned about
あんなんだ = あんな+ん+だ, where あんな (sibling of こんな and そんな) means "such a"
ん = の is "one" (as in "the red one")

