At the age you're describing, both of my kids generally started changing whatever the nap routine was that we'd become accustomed to and it sounds like your son's doing the same to you.
I'm not qualified to say whether this is normal or not, just that it's something I've seen twice now so I'm inclined to think it's normal.
If you are looking for a longer naptime, one thing you could try is making his 4 morning hours as interactive as possible from the perspectives of mental and physical stimulation. My son is 2 1/2 now and with some reliability, I can get an hour and a half nap out of him, by keeping him heavily involved in many of the things we're doing.
To exemplify, if we go out for groceries, I hand him most things so that he can put them in the cart. If we are playing together, I will make him bring me toys and make sure that he puts them away. And I will always include a trip to the playground so he can run around and burn off any excess energy remaining.
You'll need to figure out what kinds of things are appropriate for your baby since every baby's different, but you get the idea.