A few suggestions:
I would make sure there are at least 3 hrs between the afternoon snack and supper, so that he is motivated to eat. I would also keep that snakc very small.
I really think you need to wean him from that 5 AM feed, as this may contribute to less calorie needs during the day (i.e. not eating supper). Personally I would just offer water or nothing at all when he wakes at this time.
I would continue with the approach of, if you want to get down and be done that's fine, but if you want to eat you need to sit at the table (assuming that's improtant to you).
I would also really suggest implementing some sign language with him, especially signs about food and eating (more, done, different food items). Sign language in general is great for kids who are frustrated and not talking yet, and actually encourages verbal skills and decreases tantrums!!
I would also suggest offering lots of choices surrounding the meal experience. I am not talking about food items (although I would be sure there are at least 3 choices of foods on his plate) but what cup to use, whether wants to sit in booster deat or highchair, etc...
HTH!!!