The best advice I can give is start a routine, that way he will know what is coming. My daughter was an erratic sleeper as well and at about 13 mos (she is now 19 mos) we finally fell into a strict routine. First thing was to give up her morning nap, which meant she took her afternoon nap around 11:30-12 and would sleep for about 2 hours, it eventually moved to 1. Her bedtime was at 7pm when we got rid of the morning nap and is now 8pm, it will vary just have to see how tired he becomes.
Our routine for naps is we go to her room about 15 mins before nap time and hang out, I get things ready, her bed, close the curtains, turn on her fan (has to sleep with it), etc. Then I encourage her to pick out a couple of books. Then we change her diaper, and sit down and read, then into her bed she goes. She knows once she's in there its nap time or bed time and there's no getting out till the time is up. Now this was not without struggle in the beginning, but now she asks to go upstairs or into bed and falls asleep on her own.
Bedtime routine is similar, but she gets a bath, then we read then she goes to her bed and we sing a little and then it's night night time.
A helpful book to understand sleep habits in babies/toddlers is Healthy Sleep Habits Happy Child. I didn't follow this book to a T, but it was helpful in learning the cues and what to consider.
We didn't sleep for a year before our routine and now she sleeps 8 to 7 and takes anywhere from 1.5 to 2.5 hour naps everyday.
Routine, routine, routine!