"Upright sleep position. One theory on SIDS is that infants have mild reflux episodes (a small amount of milk and stomach acid regurgitates up into the throat) that irritate the upper lungs and cause the baby to briefly stop breathing. We do know that this occurs in premature babies, but these babies are monitored and assisted through these stop-breathing episodes. A baby at home alone during sleep may experience such a mild reflux episode that can trigger a SIDS event. This is theoretical, and there is much more to SIDS than just mild reflux.
We do know that the Amby keeps baby in a slightly upright angle during sleep, which decreases the likelihood of any reflux occurring. This may be one reason why babies sleep so safely in the Amby."
http://www.ambybaby.com/SIDS
Maybe its a good thing he sleeps like that