I am glad that you’ve shared this problem. Your problem is not unique, people from all over the world are experiencing this issue and they easily brush it off; but it should be taken with more seriousness than it is being afforded right now.
I would imagine that you usually have to make a lot of considerations; not only for yourself, but for your son as well. You need to consider what is best for you and your son in every single situation you find yourself in, and this is no exception.
How serious is the guy about you? Has he done more for you than just been romantic? What do you guys talk about, do you converse like adults about adult business or is it all romance? How does he feel about you having a son? And most importantly, do you see yourself building something good with him?
Most of these questions would otherwise be necessary, but in your situation they need to be answered. Unfortunately, at some particular stage, romance may not enough. If he is past that, then that is a good sign.
However, at the end of the day, he has a girlfriend. In my opinion this is never a good sign, and I am very pessimistic about such situations. I feel like if he makes you feel special which caused you to fall for him while having a girlfriend, then what assurance do you have that he won’t do that with another lady? What can he possibly say, or do, to lay to rest these suspicions? Think about it.
And on top of it, I think that at least one of you should have respect the foundation of a relationship, and I believe that it should be you (because he has proven to have none). Respect the relationship that he may have and respect his girlfriend too.
If you’re comfortable with everything mentioned above and wish to proceed, approach him. Aask him why he’s doing what he’s doing while having a girlfriend, and also what plans he has for the future. If you don’t see yourself in the plans, then I believe that it is time to let go.
I hope you do the right thing.