Our 10 mo old son just started falling asleep sitting up in his crib. When we lye him down in the crib, he fusses for a few seconds and then sits up. At first we would go in and lay him down. But he just seemed to get more upset when we would come in and then leave without picking him up. So we decided to leave him sitting until he would lye back down and fall asleep, but instead, he just fell asleep sitting up. then he wakes up after 15 minutes or so and is really upset, still tired, but no longer sleepy and won't go back down. any advice?