We have 'food stations' - at the bench (now he's bigger) at his desk and at the table. Soemtimes also on the couch (special treat). Now he's older we insist on him sitting to eat, but when he was younger he couldn't do it. So the food stayed put, but he was free to come adn go. And often stood up to eat (next to his 'station')
Standing still to eat might be your first step to getting him to sit. Maybe just try insisting that the food needs to stay in one spot (so don't follow him around with it). And if the brakfast bar works, then use it