Helping baby sit up....

My baby is a little over 6 months. Not sitting up unsupported yet--but doctor said he will take a little longer since he is a bigger baby. He is 21lbs 5 ounces and 29.1 inches long.

He does this when I sit him on his bouncer/high chair and his bath tub. But if I prep him up on a flat surface, he just kicks and leans on me.

Are we getting close to sitting up? Anything I can do to help him too?