How do I know he really wants to be there for me and our son if he never seems to put in effort for us?

Many times, honestly, the biggest indicator of a person's intentions is their actions. Do they show that they want to be there? Do they want to spend time with you regularly? Of course, it is always important to keep in mind their perspective, everyone is very busy nowadays, however, if you care about someone, you will always find a way to figure it out. If they have been showing you that they 'do not' want to be there for you, then that should tell you something about their true feelings. A conversation may be in order, which is the best way to have honest, open communication.

I think there are a million things you can read into his actions, and in a way the old saying "actions speak louder than words" are true. Still I think it is important to initiate a conversation about the topic, because it is an important one. If his behevavior botheres you, and it sounds like it does (as it should), then it is important to get things out in the open. Depending on how the conversation goes, you can of course see if his behavior and effort improves, and then you can still start coming to a decision that is best for you and your son.