How Can I Get Him to Stay Seated in the High Chair?

This is going to sound a bit odd, but we never used a high chair w/ my first son. We always fed him in his swing (he didn't like to sit still either) and then when he was big enough we had the booster seat in the chair at the table. He was fine in high chairs in restaurants too.

Have you tried a swing? How about even feeding him on the floor? Not like a dog or anything, but maybe putting him on a blanket on the floor w/ his dishes? I am certainly no expert, but I do know kids always act up more at home than anywhere else.
You said you have twin boys--does the other one sit in his high chair? If so, it might be a competition/attention thing.
Hope this helps

We ditched the highchair for our son at that age. He was so squirmy that I felt it was a safety hazard. A booster chair at the table has worked out great. We also let him climb down periodically and run around the room and come back to the table. It's really too much to ask of him to sit still for such a long time.

This came up once in an ECFE class, and the instructor suggested that when he squirms and ignores his food, you take him out. Then try again a little while later. If he squirms again, take him out again. In other words, he only gets to be up there (or in any chair) if he stops climbing and starts eating. She said kids that age won't starve themselves, and eventually their need for food will override their need to win the battle. Of course, do this ONLY if you're comfortable given your concerns about his weight. Good luck!

When my kids were little we put a little picnic table (plastic, little tykes or step2 type) in our kitchen. The kids would sit there to eat (I did daycare so this helped with all of them sitting there). I think they were a bit older than your son, but if it's important to you that he sit while eating, maybe something at his level would work.

I would recommend getting a booster chair that straps to a regular dining room chair. You can either attach a tray to it or push him right up to the table to eat. That age is really hard to get them to sit still for more than five minutes. Feed him lots of tiny meals/snacks through the day to get the nutrition in him and then cut down on the power struggle. If he wants down, let him down, but if he wants food, then he needs to sit in his booster.