Seems possible. If Jeyne does have Robb's son, I'd assume she's with Brynden Tully.
Howland Reed was with Ned at the place where Lyanna died, so he might know the truth about Jon's parents. However, he's hiding in his bog lands and there's no way to reach him.
I'd wager one could find Reed if he wanted
you to find him, though. And the Reeds seem to be on pretty good terms with the Starks (what little is left of the family, anyway).
I guess the question though is, besides Jon himself, who stands to benefit from his parentage being known? If there's someone who wants a Targaryen on the throne and not
Daenerys, it has some use, otherwise it's as trivial as Maester Aemon's parentage.