Most Helpful Guy
You should demand that your boyfriend takes you with him to get his son the next time.
Most Helpful Girl
Fair warning: I've never been in the situation myself but I have mediated over them. What it boils down to is trust and communication.
1) His son comes first. There's no buts to this, son is number 1 always and you have to respect this.
2) You must understand that maintaining a solid positive relationship with his ex is vital for the benefit of his son. The only person that suffers if those two can't communicate is that boy. You cannot be the barrier between them that causes a breakdown of their ability to co-parent. This is going to include him sharing a meal with them. You just have to accept it so that he doesn't have a reason to hide it. When he feels he has to hide that relationship, he is going to sneak around doing more and more. It is a better option to say, I understand you can't avoid it, and I trust you to not betray me. When he doesn't feel like he will face hostility from you, he will naturally be open about their activities and relationship.
3) You cannot let your own insecurities overcome the trust you have in him. He still talks to his ex not because he wants to or because he want to hurt you. He talks to her because she is the mother of his son and he has to stay in contact with her. He is being forced to stay in contact with her, he is choosing to be with you. Trust him and love him for it. And love him for being a good dad that is willing to put aside the hurt from a break up in order to be there for his son.
4) I'd like to point out he made the effort to end his relationship outside his with her. This was done when you expressed your hurt. That says he listened to you, cared enough about you to alter his life and put you above her. Did he have a meal with her, yes, did he hide it yes. Did he feel pressured to do so by you, yes. He hid it out of protection of your feelings.