Sleep training baby the Ferber method rolls onto stomach, won't roll back and gets upset, what to do?

Hello! We are on night of sleep training our 4 month old son using the Ferber method and we encountered a rolling issue we had not been prepared for. He has been rolling from his stomach to back since he was 17 days old and from back to stomach since 3 months. He prefers rolling from back to stomach and often just won't roll back as easily. He likes to sleep on his side but has been rolling all the way onto his stomach throughout the night and will sleep there a little or start whining/crying (I am not worried at all about suffocation as his head/neck strength is impeccable, he just gets uncomfortable and doesn't want to be on his stomach anymore). He is in a sleep sack/blanket, his arms are free but his feet are inside the sack (he has rolled both ways fine in this but perhaps a sack with foot holes would be easier for him?).What do I do when he gets whiny and stuck on his stomach? Do I just roll him back after the interval of time (Ferber method) and hope that he will figure it out soon?Thanks for any advice on this or sleep training in general!!! via /r/Parenting http://ift.tt/2pgmyQM