hehe they do that my pup is almost 9 months and 130 lbs ! so he really gets me tired. he walks great but when he decides its time to lie down he just does and rolls on his back like dead meat and no matter what he would not stand up.
i use a prong collar and small jerky repetitive moves would sometimes work. pulling him wont work you have to jerk the leash 3 to 4 times consecutive. or pull in the direction opposite to his posture as if you are making him stand up. or hold something he likes a treat or a ball and entice him to stand up and treat him when he does. as you walk him just make few sudden pulls on his leash when you feel he is going to stop and fall down again.