Flexibility?

So this guy that I like and he likes me back have been talking about meeting up to get nasty if you catch my drift, issue is his favourite position is missionary okay that’s fine, however he wants my legs over his shoulders.

He’s 6ft4 and I’m 5ft3 so I’m already dreading that part but I’m just not flexible enough to get my legs that high, I’m also extremely self conscious of my body

So I don’t like doing positions that could give me rolls, double chins etc, but I’m willing to try, so how do I become more flexible so that it can be better for him or how do I explain to him that I just don’t bend that way