Reraise him to 360 or more. Jacks are a good hand, but you certainly don't want to cold call him here. You've got to reraise (assuming he's a good player) to get a read. If he plays back, you're probably beat here. If he smooth calls, you've got to worry about the slowplay, but it's a less likely option. Let's assume he has aces. He raises to 120. You shove back with 360. If he's got a hand that'll beat you, he's probably putting it all in the middle. If it's mediocre, he'll probably limp and try to set. So, beware that unders board. You've certainly got to bet, but don't discount the fact that he might have tripped up.

It's nothing you haven't already noted, of course... but you've got to remember that you're very early and the opponent is PROBABLY just on a couple overs and trying to buy a pot. if it were later and you had a good read on him, you cn lay it down, but I think you have to reraise. If nothing else, you've got position on him Post Flop and might be able to outplay him there.