Should I let it go or go?

Been dating my bf for almost a year. We met on a dating app. I’m very disappointed in myself for lowering my standards enough to look for someone online. But, it’s the way now. So, anyway, we met we had a connection, he was very interested, me not so much. I eventually started warming up, and we became intimate and I’m the type that when physical intimacy happens I tend to become attached. So I asked him one night If he would want to hang out, and he told me he had plans with his kids, who were 21 and 17 at the time to

Go out of town with because he has not been able to see them much due to
Corona. Ok. Cool. Fast forward to a week later, he wants to take me out of town to this awesome place he and his kids stayed at a week Pryor. It was lovely. On the way back to our town we were talking about various topics and he slipped and said that he hadn’t seen his kids in over three weeks. He immediately knew he had slipped up. I
Laughed it off. Deep down knew he was lying but at the time I thought we were barely official and I wasn’t trying to dwell on something like that. Fast forward to now, I have developed a very good relationship with his youngest who is now 18, and it came up last night in conversation and the poor child was so put on the spot and you could see he was a little uncomfortable. My boyfriend tried to manipulate the situation into
Making his son think it was all a joke, the whole time I’m thinking… what a damn liar. So now I’m kind of upset to know it took him almost a year of lying and to know he took me to the same place he met someone else a week Pryor. Am I being immature? Should I let it go like I have the past year when I didn’t know for sure but now I know for sure and I’m pissed.

Normally I would say to let it go, it happened a while ago before anything really serious happened. The only reason I would question it is because he put his son in the middle of it and put him on the spot. Like what was the reason for that… he knows he messed up. If you want to make the relationship work I would say talk to him about it. Maybe he’ll come clean and you can express how it made you feel… and see how you feel & go from there.