B.W.
He may not be big enough to sit at dinnertime in a regular chair, how about one of those booster seats that straps onto the chair that has a seatbelt?
As for church, my boys are 4 1/2 and 16 months, and they both sit still and are quiet for Sermon, and I guess my best advice is to bring a snack, and a few small quiet activities. The boys get to pick 2 toys to bring to church. My oldest always picks his little tractors, my youngest always brings a couple THomas trains. Taht keeps them busy during church, but during sermon its a snack all the way, usually pretzel sticks, goldfish crackers, and one bag of fruit snacks each. Fruit snacks are A HUGE treat in our house, so saving them for occasions like church really makes them appreciate it. :)
ETA--2 1/2 is WAY too young to even think about ADHD! Seriously. He's just being a normal 2 1/2 year old, and he just needs to learn the boundaries and 'rules'. :)You'll soon find something that works, but like another poster said its also a phase.