Does he graze a lot throughout the day? DD was generally a pain in the butt to get to finish a meal within a reasonable timeframe until we started restricting the near-constant between meal snacking and milk drinking as by the time mealtime rolled around she was already full and not interested in eating any more. So for about an hour and a half to two hours before dinnertime it's a rule of nothing but water. Not an excessively long period of time really, but enough to let their tummy settle and register a feeling of hunger before mealtime.

I'd also second the suggestion of sitting up at the table as a family for mealtimes without distractions (no TV on, pets out of the room etc), with mealtime being over when mum (and dad if he's there) are finished, everything gets packed away and that's it. The rate at which you eat then becomes a visual cue of "oh, mum's nearly finished, better get a wriggle on or miss out". It feels mean to draw that line the first couple of times, but by being consistent they soon work out where the boundary is and that they DO have a choice - eat at mealtimes or wait, even if that means getting hungry.