How to tell him I want to sit on his lap?

So I want to sit on my boyfriends lap or thigh, but whenever were together he never sits down he always stays standing up when were talking. I even sit down and like make room for him next to me but he still doesn't sit down. And I don't want to be like "Sit down." Because that would just sound weird. How can I get him to sit so I could sit on his lap or thigh?
How to tell him I want to sit on his lap?
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