Hi J.,
Congratulations on giving your son (and yourself) the amazing gift of breastfeeding - the longer you nurse your son, the more the protective affect for you and him (decrease in cancer rates for both mom & baby over the long haul!). I know it might seem like your son is nursing "solely for comfort" but if he's getting ANY milk from you at all, he's still benefitting from the immune factors in your milk, even at 15 months old:
"Some of the immune factors in breastmilk have been shown to increase in concentration as the baby gets older and nurses less, so older babies still receive lots of immune factors. So as a baby starts to nurse less (weaning) and milk supply decreases, the concentration of immunities increases. This isn't age-dependent, but depends on the amount of milk that baby is removing from the breast. [source: Goldman AS et al. 'Immunologic components in human milk during weaning.' Acta Paediatr Scand. 1983 Jan;72(1):133-4.]"
http://www.kellymom.com/nutrition/milk/immunefactors.html
Also, about your son not being too interested in cow's milk, please don't be terribly concerned by this, especially since he's still nursing 3-4 times a day - this is more of a cultural habit (to feel like we "need" to drink cow milk to be healthy) than it is a nutritional necessity:
"Many nursing moms are told that they must introduce cow's milk at a year. Your nursing toddler is already getting the best milk he can get - mother's milk! Breastmilk has a higher fat content than whole cow's milk (needed for baby's brain growth), and all the nutrients of human milk are significantly more bioavailable (easily absorbed) than those of cow's milk because it is species specific (not to mention all the components of mother's milk that are not present in cow's milk).
There is no need for additional milk or (or the equivalent nutrients from other foods) as long as your baby is nursing 3-4 times per day. Cow's milk is really just a convenient source of calcium, protein, fats, vitamin D, etc. - it's not required. There are many people in many parts of the world who do not drink milk and still manage to get all the calcium, protein, fats, vitamin D, etc. that they need."
http://www.kellymom.com/nutrition/solids/toddler-foods.ht...
Now about weaning... :) From your request I can't quite tell how you feel about weaning your son, so I'm not sure how to respond. I'd be supportive of whatever your feelings are, but my suggestions about "how to wean" would probably change depending on your current feelings towards your son's nursing right now. It would help to know things like... Is weaning something you're asking about because you think you're supposed to be done by now? Do you have strong feelings either way, or are you ambivalent about weaning? For example, are you going nuts and can't wait to stop nursing? Or are you fine with continuing to nurse as things are right now? Or do you like nursing still but would be happier if there were only 1 or 2 nursing sessions a day?
However you're feeling, you have lots of options. :) For your physical comfort (lemme tell ya, clogged ducts, breast infections, mastitis? NOT fun - recommend avoiding them!) as well as that of your son's physical and emotional comfort, a gradual weaning plan of some sort that has some flexibility built into it (allowing for a break from the weaning schedule in the event of illness or major life changes) can make weaning a really pleasant, sweet experience for both mom and baby. You can do partial weaning - like eliminating one nursing (for example, night weaning or perhaps you just aren't loving that morning nursing anymore... slowly cutting that out). Or you can gradually shorten the time of each nursing session ("you can nurse for as long as the ABC song") or you can set up something like a special nursing light (that only you can turn off and on) and explain to your son that you guys can only nurse when the light is on (and help him set his expectations - will that be the 3-4 times a day? or are you only going to do it before sleep?). There are *many* other clever ideas out there for gradual weaning from the breastfeeding relationship.
Some other great links on weaning your breastfed baby:
http://www.kellymom.com/bf/weaning/weaning-partial.html
http://www.kellymom.com/bf/weaning/how_weaning_happens.html
Personally, we went through several stages of weaning. At one point I night weaned her gradually with the help of my husband (maybe 14-18 months old?), and then over time I remember saying to myself "I still enjoy nursing when it is 3 times a day or less" so I think we made a rule to only nurse in one, specific, special chair that I chose to go sit in (although I did try to be respectful of her wanting to nurse and would try to make her needs and mine coincide - otherwise I played the Big Distraction Game! like "want a snack? let's play chase" etc.). And finally I remember being somewhat surprised when my daughter actually weaned completely because it was such a gradual process that she eventually was nursing only once every other day (or skipping a few days in between) and only for a very short minute or two... so the last time she nursed, I wasn't aware that would be The Last Time! I was pregant with my 2nd child at the time and she was almost not quite 2.5 yrs old. And now she's almost 4 yrs old (already!) and it feels like ages ago that she ever nursed... Ahh, treasure the time because it really, truly flies!
Best of luck to you, and again, it's so wonderful to hear of a mom/baby pair who has had such a successful breastfeeding relationship. I hope weaning is a pleasant experience for you both!