Too much depends on personalities we don’t know to give a definite answer, but even if it’s deserved, being a jerk back rarely improves the situation.
Start with replacing “jerk” with “coworker” in your question. This establishes a baseline for how you should at least normally treat people. How you are viewed by others will be heavily influenced by how you, in turn, treat others. So there’s at least an argument for being the bigger person here.
Next, consider what could come of it. Some bullies become best friends when the olive branch is offered. It may be his own insecurity leading him to be as he has and your willingness to look past that could mend the relationship a lot and make everyone more productive.
If you’re concerned about being walked over, then lead in with that discussion. “[Jerk’s name], I’m quite willing to help. First off, I need to clear something up. I asked a similar favor of you weeks ago and you seemed to go out of your way to belittle me. It may have been unintentional or maybe was just a bad day. Whatever the interpretation, things will go better for all of us if we can cooperate. Now let’s take a look at this issue.”
Similarly, you might consider covering yourself with your manager first. It’s a similar conversation, mainly just laying out you’ve had a serious challenge with this guy. You are planning on moving forward and helping him, but you will not be able to if he continues to treat you in the same fashion as before.
Add-on based on your comment
It sounds like this is just how the guy treats you.
The feelgood answer is “You should’ve thought of that before you treated me like an absolute jerk. Good luck finding the answers on your own little island.” Best case scenario here is everyone else feels the same about him and it puts him one step closer to leaving. It’s definitely understandable, probably even deserved. But what if others didn’t perceive the same treatment? What if he goes to his manager and explains what an absolute jerk you are for not answering a simple question? He may not be right, but sometimes the powers that be may not hear it the same as you perceived.
I’d start with your manager as above. Make it clear you’re being the bigger person and give him the opportunity to tell you it’s not your project and you shouldn’t get involved. At this point, you go into the next conversation with your manager on your side. Then go to the jerk and tell him your view on the situation before helping him.
2nd edit (TL; DR)
I apologize as it is not my intent to be rude but I fear this will seem to be.
You seem to not be asking a question more than wanting us to fuel your rage. Pettiness and combativeness rarely make for a better overall work environment. Yes, it is within your capabilities to be as ignoring or mean as you wish. This back-stabbing may work in some cultures and corporate environments. In healthy ones, it does not.
I believe in letting my work speak and bringing my manager solutions -
That general philosophy is good. But you may misunderstand what management wants. Keeping your manager in the dark about a potential problem can be much worse than if you diffused it from the start. Your job is rarely to decide what’s best for your manager. It is to keep him informed of the good and bad. If you find a bomb on your bus but don’t know how to diffuse it, would you not tell anyone? Sometimes it’s better to get problems out in the open—even if you don’t know how to be the hero.
His manager is not my manager.
I’m sorry if I indicated I assumed he was. It’s plausible if you don’t open this up to your manager, that the day can come where he makes you look like the bad guy and his manager comes and talks to yours about your insolence.
He's not using any work emails to ask me questions but doing it in
I am unclear as to how this is germane. Yes, recorded proof may help, but regardless, turning this into a finger-pointing session, documented or otherwise, rarely makes his manager OR yours happy.
Let him gossip about how mean I am. […] At the end of the day,he'll
get tired and go and bore somebody else.
And possibly tarnish your reputation along the way, too. Maybe not. But only you can decide if it’s more important to you to be unbending than cooperative.