FIL touching my belly

Ok so I need a little advice. My FIL has a habit of putting his hand on my belly uninvited, it's almost like his hands are a magnet to it. My SIL said he did that to her and would leave it there until she awkwardly walked away. I don't mind family touching my stomach but I'd like to be the one to invite you to feel kicks. My family and MIL are very respectful of this, my own father hasn't even tried to touch my belly yet. I'm not a rude or blunt person when it comes to my in laws, i love them and dont want to step on any toes. what is a nice way to let him know to please stop touching my belly unless I invite you to?