Just ask him. No harm in asking. Tell him you didn't really think about it at first, but "it sounds like fun, if the other women are going...." etc.
Perhaps he didn't ask you to go because you are pregnant, and have an 18 month old son....and certainly it's not healthy for you to be in bars while pregnant. Also, if the men are going basically to let loose and have fun... it's not going to be easy to do that if his whole family is with him. That's probably what he is thinking MAYBE.
I wouldn't project and put negative thoughts in your head... or assume things about him as to "why" he didn't ask you to go. Just tell him in a nice way.... "Honey, did you want me to come too? Or not and just want to have a boys outing??? I notice some other women are going...."
As far as him having a "reunion" with the guys...there's nothing wrong with that conceptually. Just tell him to 'take care and be safe....don't drink and drive, and remember you have a pregnant wife and children at home...." but say it in a nice & caring & non-possessive way, so he knows that you are not trying to be nagging.
As far as him going to bars and strip clubs... well men are men. You NEED to trust your Husband. For me... even if my Hubby goes out with the guys occasionally (and yes strip clubs).... I TOTALLY trust MY husband even though I don't know ALL 70% of the other guys. I "know" my Husband... I trust him explicitly. I know he knows how to handle himself, I know he is not obnoxious or immature, I know he is not easily influenced and can make his own decisions, I know he is respectful of me and our children, I know he can make good judgments even in a bar, I know he will not put himself at risk, I know he is not a 'follower', I know he can stand up for himself, I know he puts his family and me first, I know he will not engage in risky behavior, I know he will not fool around, I know he will not do anything stupid, I know he will not break any laws, I know he CAN be trusted. Therefore, I do not feel insecure if my Husband goes out with the boys, or away from home, or is around other women.
Now, how about you? How about your Husband? Can your husband handle himself and 'behave?" THAT is the question you need to feel secure with.
At least your Husband asked you first if he could go. He didn't have to. You said yes. If you are unsure about it, just talk to him. At least your Husband ALSO said that the other guys are taking their girlfriends.... he was honest about that.
Just talk to your Hubby.... nothing wrong with that. You are pregnant and will be at home by yourself AND with your 18 month old son.... tell him your concerns. Be open and honest... you should be able to communicate anything with your Husband.
Tell him it does not "feel" right for some reason.... ask him why he didn't invite you etc. Just be open... maybe it is just your insecurity about it and being pregnant. You should certainly be able to discuss it with him. And certainly, he is only a cell-phone call away, right? He should be able to answer the phone anytime. Don't worry, unless he has a history of disrespecting you and your children.
Take care and good luck,