Make sure you wean slowly, if at all possible since you have to start medicine.
It took us about a month to wean. I slowly dropped nursing sessions. First I dropped the afternoon sessions, then after she was fine with that, I dropped the morning session, then lastly the last session before bed time. My daughter was 14 months when we weaned, so she was only nursing about 5 times a day... so I waited about a week in between dropping each session. It might take longer for you depending on how much you're nursing. Since you're weaning before he's 1 year, replace the nursing sessions with formula bottles. You might want to try mixing breastmilk with the formula if he has problems (though you said above he hasn't yet).
If you have to stop nursing sooner than that, you can pump and dump the milk, and then just slowly wean from the pump.
With both of the ways I mentioned above, your body will hopefully be able to adjust to the lessened demand and produce less.
Make sure you're drinking plenty of water to help clean out your system. You can also try decongestants (like sudafed) to help dry up your milk... might want to check with your doctor on that one though.