Dogs generally don't know what to do just because you present to them even if they're unfixed. When he humps your foot you can wrap your pointer finger and thumb around his penis behind the lump (flaccid knot) and saueeze gently and stroke back and forth somewhat rapidly. Get him used to humping you = sexual gratification. Then you can try to get him to hump your body. That can be the hard part. Try to pay attention to the circumstances that make him want to jump your leg, like perhaps it's him being energetic or you playing with him, and try to recreate that occasionally. Then when he normally would hump your leg you can present to him and hope he decides to try something new and grab your hips.
He is a fixed dog so you can expect that sex with him will likely not work out. His knot may not get to full size, may be overly sensitive once it grows, or he may just not do anything at all. It's still worth a try to see if your dog might be the exception.
