I don't know if it will work for you, but for DS2 I used to go in his room a few minutes before his sleep cycle was up (his was 40 minutes) and watch him (I kind of hid so he couldn't see me). As soon as he stirred (he'd sigh and move his head) I'd pat his tummy, still keeping out of sight as much as possible. He would sometimes go back to sleep, sometimes not, but it worked for at least one of his naps every day. I persisted with it until eventually he started working out how to go back to sleep by himself.

Not sure if you've already tried this, but I thought I'd post it anyway .