Practicing sitting in crib

hodgie

DS has recently figured out how to sit up in his crib but hasn't quite mastered going back to laying.  So at naptime and bedtime he'll be quiet for a bit and then start crying.  I find him sitting up, practically asleep but crying.  It's a pathetic sight.  When I pick him up, he pretty much passes out in my arms and I can put him down quickly.  He still sleeps fine, just wondering if anyone has any tips for teaching him to lay back down.  We do a lot of floor play but it hasn't seemed to make a difference.  I don't remember going through this with DS#1...maybe I just blocked it out!