Anna, I also believe that children should sit with the family to eat. However, trying to keep an 18-month-old in one place can be difficult. Their legs and brains are running twice as fast as the rest of us!
Your son is old enough to understand that you would like him to join with the family, and that food must be eaten at the table, but he also needs to be given some flexibility to not have to sit as long as everyone, at least at this stage. Tell him that you like his company, offer him a coloring book or activity while you finish your meal. If that doesn't work, let him go on his way. After all, in his mind, if the table is to eat, he should be able to leave when he is done. Also, don't offer him food if he's not in his chair. If he asks for food, place him in his chair. If he tries to leave with the food, take it from him. Be consistent, as toddlers learn through repetition. He will get the idea after a few incidences.
Parenting advice is given as a suggestion only. We recommend you also consult your healthcare provider.