I think his comments at Barton cottage after Marianne's accident are a sort of continuation of his and Mrs. Jenning's raillery.
I think he was enjoying all this, without thinking about the embarrassment caused.
He is not too quick in getting people. Marianne's rude remarks go right over his head, and he laughs as though he understood.
I doubt that he would try to talk about it to Brandon, let alone notice any attraction on his part.
Elinor of course begins to notice his attraction for Marianne.
I too think he would feel down about it, maybe that's what Elinor noticed in the first place.