hi kirstym
i use to have TERRIBLE problems with short naps. they still happen occassionally but I am atleast getting one good nap a day, which is great for us!
When I was having problems with short naps, I was told that 30 min naps mean OT and 45 min naps can be OT or UT. Since your LO was taking great naps before and has now switched to shorter naps, I think you definitely should extend the A time, he's more than likely UT. I was told to extend the A time by 10 minutes and try that for a few days bc sometimes it takes awhile for them to adjust. If that doesn't work, maybe try extending it another 5 - 10 minutes.
I hope that helps some. I'm sure one of the moderators will be able to add some advice too! Good luck and stay postive! I know how frustrating short naps can be, but hopefully you will have it sorted out soon!