I really feel for you, and I know what it is like to be involved with someone playing the "secret" game...I must tell you that the relationships that I have been in with that type of scenario, didn't work out. And, while I was in them, I was miserable.
You really do deserve to have someone be honest and open with you, we all need that. It really is the only way a relationship can work, otherwise there is a huge lack of trust.
I would suggest sitting him down and telling him everything you have been thinking, your insecurities, how you feel about him, and what you want. You deserve honesty, and you should have it. If he can't give you that...and refuses to try then you don't have much to go on. But give him the chance to tell you what is going on. Don't be afraid to ask for, and get some accountability from him. Set some good boundaries with him, and let him know what you can and cannot deal with. Like, there should be a boundary about him not coming home...that really is unacceptable within the confines of his relationship with you. That is disrepectful towards you, even if nothing is going on with the ex. He can, and definately should, see his baby, but that can be done during the daytime. And if he doesn't have enough gas to get home, the ex does have a phone doesn't she? Sorry, but sounds like a huge "red flag" to me....
I know this sounds harsh, but it sounds like you are unhappy living in "hell". Let him know, give him a chance to fix it...and at the first sign of his "secrets", get out. He is young and so are you, there are sooo many great guys out there, that aren't perfect, but can be at least honest about their imperfections. And if you do break up with him, remember this, while it hurts for a little while, it is so much better that being in a relationship that is hurting you indefinately. Trust me, I really know about this.