Hoyden, you don't need milk if your son is getting calcium from other sources, especially dairy. Yoghurt and cheese are just as good as milk in that respect. And if you're worried about fluid intake, give him juice - diluted - and dilute it down over time until it's just water.
You've done an excellent job breastfeeding until 17 months, most people don't do it that long and no one should make you feel guilty about wanting to wean him from the breast.
So, that having been said, I completely agree with aloha and runragged. Basically, if you want to wean your son, then wean him. It will be hard for a few nights (a week or so?) but with my latest, my husband took over the nights and it worked out fairly quickly and without problem. It's harder if you have to do it yourself (ie refuse to feed him from the breast as opposed to your husband giving him a substitute -- bottle, dummy, whatever). I stopped breastfeeding the youngest at 18 months or so and the eldest at 2 years.
"Cruel", bobthebaby? Sheesh. Y'know the holy Dr Sears accepted advertising from formula makers for his website. Hmm... gotta wonder where else the man fudged a bit. And I know all about milk allergies lactose, being a sugar, can give you digestive problems but you cannot be allergic to it. It's the proteins. (And don't worry hoyden, I wouldn't have done anything to put my second child at risk my first child is the one with the allergies. The child I breastfed for 2 years. I'm skeptical about the hype regarding allergies and breastfeeding.)
Wean at will. Feel no guilt. Sleep well and long.