The grass is always greener on the other side of the fence, now isn't it?
Before you think of throwing away what you have, think HARD about what you would be giving up.
I have been with my husband now for 10+ years, and believe me there have been moments when I want to just throw my hands up in disgust and walk out the door. But then I remember all the kind, considerate things he has done for me, and I remember how hard he works to provide me and my 2 yr old son with a happy, stable home.
Marriage is a choice you make every single day. I keep choosing to be with my husband, and he keeps choosing to be with me.
I assume if you're engaged to this guy, that you love him, and that you have a good, solid relationship with him, built out of trust and friendship, not just lust etc. With a solid foundation, the relationship is WAY worth working hard to keep going strong. Marriage is not always easy, even when it's good.
Now, if you're engaged to him just because he's your son's father, and because you were/are attracted to him, but you don't have the deeper feelings of a connection with him, then both guy #1 and #2 are on a more level playing field when it comes to how invested you are in them.
If that's the case, what makes you think the other guy would be any better for you and your son or have anything more to offer that the one you're engaged to? Either guy, YOU still have to put in the time, energy, and will to commit to him and to keep choosing him, day after day after day.
Either scenario, I personally would advise you to stay with the one you're engaged to, and yes, I do say that partly because of what an impact it would have on your child to change men now (and likely again in the future...)
By the way, by not telling your fiance about your indiscretions, you are not being fair to the other person who needs to be able to be part of this decision-making process. (My husband decided he loved me enough and was invested enough in our relationship that he chose to stay with me after I made a bad decision involving another guy. But I have also never let this happen again. If I did, I certainly could not expect my husband to feel the same about me again.) He needs to know that something is going on; not necessarily all the details, but at least what is happening with you emotionally and the conflict you are feeling. He needs to be able to make a choice too: to give you another chance or not.
One more thing: the other guy you're seeing; if I read that right, he knows about your fiance. Anybody who will date someone who is already nearly married is not going to be a reliable partner anyway. If you end up with him, I wouldn't be surprised if he ends up cheating on YOU.
Think really hard about what is best for you and your son. Don't let infatuation and romance and excitement sweep you off your feet. Use your head as well as your heart.
(Sorry this was so long- I feel really strongly about this one.)