Mine does that too when I sit for a long time. Especially when I drive or on my baby shower when I was opening presents. I sometimes stand up and move around to get my baby out of that position
My only suggestion is to tickle the baby to get him to shift his position when he's doing it, and make sure you keep your calcium levels high. The bones definitely ache more when your calcium is low.
My son pressed so hard on my ribs with his feet that they ached all the time, once he was big enough. He was born with pretty bowed little legs, too. I thought he'd crack one too. It was noticeable after he was born -- when he was nursing, he'd stretch out and just lean his legs into me. Made me laugh then because I could see in the daylight exactly what he had been doing in the uterus.