Suggestions on potty training a toddler who is almost there

Hi forum! I am trying to potty train my 20 month old. He tells me when he is about to pee and asks for the potty and sits on it, all cheerfully. He tries to imitate pushing but the only problem is that it never actually happens. He sits there for five ten minutes and gives up and then stands and does it. This has been happening for more than a month now.

There is no doubt he is ready but unless he actually does it once I don't think it's happening easily. So the question is what to do for the final push? I think he is too used to doing it standing and that's the issue.

How can I actually make him do it sitting there? Any reading sources? thanks!