I have a partner with a 2 1/2 year old in a similar situation. My sense (and I ma no expert) is that an occasional family dinner is a good thing and should not confuse the child. At 5 yo he really doesn't get the nuances of how people could or should be when they are no longer married. He doesn't know that often parents do not interact, he will know wht he sees. I think it is important for him to see you and your partner together as the son needs to see his parents having loving relationships...accordingly it would be best for him to see his parents getting along, sharing what they do have in common, and that if of course HIM!" This would be a challenge for me as I am sure it would be for many, but I would work at it for my partners and his sons sake. I also think it would be even better is sometimes the "family" get togethers would include YOU!