Maybe just clock watch a couple of his naps and work it out from there. My DS2 would wake up exactly 40 minutes (how did he always wake on the dot?) after I put him down. I would look at the time when I put him down and then a few minutes before the 40 minutes was up I would creep into his room and stand close by. As soon as he went from being completely still to slightly stirring I would start patting him rhythmically on his tummy. This seemed to help him make it to the next cycle. I won't lie and say it worked every time, but the hard work did pay off!

I hope it works for you too if you decide to give it a go