Why were you helping his mom cook in his parents house? Was he helping his dad fix the roof or was he “helping” his dad watch sports?
On the condom in the car issue, like others have pointed out, it doesn’t seem like he gaslit you. He did come up with a pat response, and that could be because he is a habitual liar or because in his mind it was close enough to plausibility.
Other responses have claimed that he lied to you. He may have, and you may know that somehow, but your readers don’t.
It is true that if this one single incident were part of a pattern of cagey behavior, glib responses that are easy to check for veracity, deceitfulness about relatively innocent things (“I did load and turn on the dishwasher, but my buddy Joe borrowed some dishes and probably turned it off.” instead of “Sorry I forgot, I’ll go do it now.”) etc., your gut feeling would be probably justified.
Regardless of the truth behind this one incident, if you didn’t trust this person you didn’t trust this person. So you could have worked it out together (not the infidelity, though plenty of people work that out as well) or you could leave, as you did.
Or it could be that you have trust issues. Have you explored that option?