Hello
If he wakes up but isn't crying, I would leave him be. If it's at the beginning of a nap, he may not be tired enough yet and you could look at extending his A time a wee bit. Does he go to sleep if you leave him?
Regarding the end of a nap-depends on how long he slept..if it was a full length then I would let him lie/play there for a bit so he gets used to the idea of being able to entertain himself for awhile, but then start your day.
If his nap was on the short side, leave him and see if he goes back to sleep.
When you ask about him waking up fussing every 15 mins, are you sure he's actually up? Some babies make a lot of noises in their sleep. Again, I would leave him unless his noises turn into cries.
And in response to your last question, I'm not sure....perhaps this is actually around the 45 min mark of him being *truely* being asleep and therefore he's coming out of a sleep cycle...?
Hope some of this helps.