Good for you in keeping with the breastfeeding!
I have a son who is 3.5 and a daughter who is 14mo. I also have a similar parenting style--attachment parenting, co-sleeping, extended breastfeding, no "cry it out", etc--so I get where you are coming from about not wanting to deny your son nursing. I know my daughter isn't ready for that yet and neither am I. A little background into where I'm coming from--My son and daughter were both excusively breastfed. My son wouldn't take solids at all till his 1st birthday and even then not too much, so I understand where you are coming from there. I always felt so much better knowing that he was still getting the good milk that he needed, even if he wasn't eating much in the way of solids. He nursed whenever he wanted during the day and still woke to nurse during the night until he turned 2. I also was worried about not being able to get pregnant again when I wanted to, but my period returned at 18mo, even with he son still nursing extensively. I got prengant when my son was 21mo and he continued to nurse until he was 2.5, stopping a couple ofweeks before his sister was born. He completely self-weaned (I did nothing to encourage it because of a lack of milk toward the end of my pregnancy. My daughter is now 14mo and still nursing up a storm, including 2-4 times during the night.
I'm assuming that, since you're interested in weaning specifically to get pregnant, that your period hasn't started again. However, you will probably find that, even if you keep nursing unrestricted, your period is right around the corner. The average for the vast majority of women is 12-18mo of no fertility even with unrestricted nursing. So you can keep nursing and still get your period back, most likely soon. And, once your period does return and you get pregnant, nursing while you are pregnant is fine. (It can be uncomfortable for some women, but not unmanagably so.) Many women who do this also go on to "tandem nurse" (nurse both a toddler and a baby)
If your period has returned already, there is no reason you can't get pregnant while continuing to nurse.
If you do find that you do want to do some weaning to force the return of your period, you may want to try just night weaning. The return of your period is tied into how long you go without nursing at a stretch, so most women find that they are able to force the return of their periods just by night-weaning (not nursing during the night) and still are able to continue to nurse all day. If you are also interested in day weaning, the kindest way to do it is to eliminate one nursing at a time, eliminating a new nursing each week or so, so that you are gradually cutting down. Start with the nursings that your son is least attached to and save the ones that he is most attached to (bedtime for most kids) as the last ones to cut out. You can also try shortening the length of nursing sessions (letting him nurse for 5min, rather than 15min, etc--though most toddlers nurse for short bursts anyway!) If you do decided to wean him before he initiates it, be sure that you give him lots of extra cuddles and loves so that he doesn't miss out on the emotional part of your relationship that he fulfills through nursing as well as the actual milk. And remember, if you start weaning and then decide it's not working for you and your son, you can always start letting him nurse more again.
I know this is a tough decision to make, but you sound like a wonderful, compassionate mom who is very in tune with your son's needs, so I'm sure that, whatever you decide, you will make sure that your son's needs are fulfilled, in one way or another!