You will feel it if he gets a boner when you straddle him sitting. If you wanna make sure to give him one, then keep working yourself down more and more until you you know your crotch is pressing on his genitals. Keeping this up for only a short time should produce the boner you want from him. Noo, heck no it won't hurt! It might feel a little uncomfortable for him depending on where his boner has worked itself but nothing for you to worry about. Besides, you'd want him to feel you and know that you're aware of what you've done. lol. Guys l ike that so just have fun.