L.B.
A.,
If he wont sit in the seat, you need to let him know that you are not going to hold him. if i were you if he gets out of the chair, i would let him out of the chair but i would leave the table and not hold him at all. after a few min try putting him back in the chair and see what he does. if he still will not sit, I would then sit him else where in the room while you eat, to let him know you wont hold him while you eat.
Good luck
L.