I think at this age, i was told, that they start to like to want to feel like a big person.. in that, they want their dinner plate to look like yours (not cut up and all fansy, but everything the same) and perhaps even to sit in a chair like you. Have you throught about upgrading him to a booster that goes on your normal dining chairs? Maybe if you take that approach with him, he might be more interested?

Good luck.