I feel like his shaking your hand from a seated position might be percieved as bad manners. I wouldn't feel unimportant or belittled if someone didn't stand when I stood, but I might find it odd and be puzzled. The conversation needs to be about the feelings you have, not about his actions. He doesn't have to explain why he won't stand, but maybe you can explain how it feels when he doesn't. Quite possibly it's a battle you can't win, but it might lead you to that young place and allow you to express those feelings.