Sitting up

Emily

My son will be 9 months old tomorrow. He can sit up using his arms in front of him to hold himself but does not sit straight up by himself yet. He loves to roll everywhere. He gets a lot of time playing on the floor and we practice sitting up a lot. He would just rather roll around than spend time sitting up. Should I be worried? His 9 month check up is next week so I will discuss it with his doctor then.