I've had SO many sleep problems with both of my little ones. Nursing them down for naps and bed is so effective when they're tiny, but it sure as heck doesn't work anymore when they turn about 15 months! My little one is 16 months now and starting to have trouble with naps and bedtime (argh), just like my three-year-old did. Now I'm having to pull out all the tricks I used before to help my little baby get to sleep. My tricks are much more effective at bedtime than naptime, though, I still can't figure out naps, darn it all! I like the idea of a pacifier -- most of my friends who have kids who don't have any trouble napping have kids who use a pacifier or thumb (or at least a special soft soothing toy). Also, get really good black-out curtains (cheap at Home Depot or Wal-Mart). The curtains will help with naptime and at bedtime (all those car lights making pretty designs across the ceiling can be distracting).
If your little boy is pretty adept at climbing, you might even try setting up a low big boy bed (with a rail to keep him from falling out at night). You might think this is going in the opposite direction, and it COULD be... BUT, if you have a kiddo who has trouble sleeping, it's not going to get any easier until they learn how to put themselves to sleep, even when they have that choice to get up. You can choose when to fight that battle, now (while you still have breastfeeding on your side) or later. So, nurse your baby right after dinner, then play, brush teeth, read, etc. Then put your baby to bed in a big boy bed with a soft soothing toy. If the baby gets up, you take the toy and put him back to bed. Hopefully, he'll be upset (I know that sounds cruel, but it's only for a few seconds!). Then go back in to his room and praise and comfort him if you find him in bed and give him back his toy. You can sing songs outside his door, play soothing music, etc., but less petting, rocking, etc. at bedtime. You're doing all the work and he's not learning how to do it!
But don't worry too hard about naps -- if your baby doesn't get them, he'll still grow up OK. A resting time of reading books with you and cuddling can help both of you get the rest you need in the middle of the day. In fact, you may just want to resign yourself to a resting time with reading for now, because you may find it (as I did) less of an emotional roller coaster when trying to guess whether the baby would nap or not. And then I found that bedtime was much easier without the nap in the middle of the day! Well, try the black-out curtains, paci, and soothing toy first! Oh, and when you wean, try a sippy cup of warm cow's milk at naptime while you cuddle and read -- I've heard that works, worth a try!
Edited to say that some rocking, petting etc. is OK, use your judgment, but try to remove yourself from the room while your baby is still awake.