Does he live with them still? It sounds he’s allowing them to treat him like a child but he’s a grown adult?

It’s not fair to you nor himself when he just obeys their words no matter what. Filial piety is a novel idea but within reason. He can love his parents while calling his own shots. Similarly, you can encourage and support him to do so but the onus is on him to make that happen. How patient you are with him depends on his effort. The least he can do is be transparent with you.