There's no hard and fast rule on this. If he's not that interested in a mid-day meal, then it seems like it would be a good idea to eliminate it now.
Halo was on 3 meals a day for a few months, but I don't recall how old she was when we switched to 2 meals. I'm pretty sure she was under 5 months old though. Keefer was on 2 meals a day when we got him at 9 weeks old. He did get a lot of training treats during the day, so he wasn't really waiting 12 hours between meals.