Hello. While I do certainly see your point in many of these things, one thing to remember, is they have a son together, so there will always be some sort of connection. You did not specify the age of the son, but you said they went to Vegas and also to Disneyland, so I'm guessing he isn't 18 yet. If he is inviting her to dinner, she is staying at his house, he doesn't allow you to be around his kid, I would certainly be concerned. He may or may not have any idea of how his actions are being interpreted by you. He may in fact be over her in a romantic type of way, but is still friends with her. I have seen MANY cases where divorced people remain close friends and yet still date. Where the issues would lie, is where he is spending more time with her, making her the priority over you, etc. Does he show a controlling type of personality with you ? He sounds like he could be the type of person who likes to have everything and have say in that everything. If he is still concerned over her work schedule, involved in these types of things with her, unless it is because it directly affects their son, it is a concern.
I might not go so far in saying he still has feelings for her, but he certainly hasn't let go of the control he would like to have over her. He may be "all for you" but you need to explain to him that it certainly does not feel that way and that you would appreciate if he paid you as much attention as he does to her. 5 years should be enough time for him to cut emotional ties with her, for certain. If you are just "finding out" things about him and he is not telling you these things...yes, you have every right to be upset and concerned about his behavior and intentions. I have no doubt that he will most likely paint you as a jealous person if you confront him in any type of angry way. You need to explain to him how he makes you feel, instead of accusing him of things, which he may or may not be feeling. His actions are not those of a man wanting to move on and to dedicate himself to someone else in the future. BUT he may also not be sure of how he should act. Cutting ties with an ex who is the mother of his son may be something he isn't equipped to handle properly. He may honestly not think he is doing anything wrong. He may not be treating you in a vindictive way on purpose. I do agree he is overboard with his continued attachment to her as far as the dinners and paid rent offer go. Do you live with him ? I might advise you to carefully and nicely explain to him that his actions are not showing you that he cares for you as much as he says that he is. Tell him ow much it hurts you that you find these things out about how he is still pursuing her inclusion in his life and how that things really need to start to change if you are going to stick around. Make sure you show him understanding about his son and that you would never come between their relationship, but that you really don't think he understands how his actions are affecting you. I don't think it's as simple as you or her. I think he likes having both on some level and he will continue to have both until he is forced to see that he can't have it that way and still be committed to the future. You do not deserve the back seat. Ask him why you cannot be around his son. What is his answer ? Perhaps you should consider backing off of him for awhile to show him that he needs to work for you and not for her.
If he slept with her, I'd say here is your answer right there. There is your proof. He is lying to you, cheating on you too, yes it is cheating if its with an ex ! He won't let you around his kid because of ruining his chances ? I'm fairly certain you know what you need to do at this point....He is obviously just pursuing her and leading you on with no desire to break it off with her. With this information, you should probably consider moving on and finding someone with not as much baggage, someone who isn't lying to you and someone who will welcome you into all aspects of their life.