Hello, welcome to the boards
Please correct me if I'm wrong, but it looks like your routine is something like this:
Wake: 5.30am
Nap: 8-9am
Nap: 12-1.30pm
Bed: 6pm
That gives him 11 1/2 hours at night, and 2 1/2 hours in the day. That is actually not bad at all, you are certainly on the right track, and, if your lo was OK on it, I'd say keep doing what you're doing. But, you say that he's cranky during the afternoon, and I think that's probably b/c he's awake for 4 1/2 hours in a row, which is too long for a lo that age.
What I think you need to do is space out the naps as evenly as you can, and extend them as much as you can.
So, if he wakes at 5.30am, instead of putting him down at 8 am, try to extend his awake time. Don't do this all at once, though, it's best to do it in increments of, say, 15 minutes, so he doesn't get overtired. Eventually, you will be able to get about 3 hours awake time out of him, so he will be going for his morning nap at around 8.30 - 9am. If you also do this for his afternoon nap, this should help space out the naps more evenly, and keep him going until bedtime.
At the same time, have you tried extending his naps at all? A good nap time to aim for would be about 1 1/2 - 2 hours. There are some great ways of doing this, have a mooch about on the boards to see what others are doing, and have a look at these links for some good tips:
http://www.babywhisperer.com/forum/posting.php?mode=reply&t=49697http://www.babywhisperer.com/forum/wake-to-sleep-for-naps-vt43311.htmlhttp://www.babywhisperer.com/forum/how-tracy-helped-josie-and-her-family-get-past-45-min-naps-vt649.htmlhttp://www.babywhisperer.com/forum/special-sleep-interview-with-tracy-hogg-vt3187.htmlHope I haven't overwhelmed you
Any difficulties with any of this, we're always here!