M.M.
Milk Production responds directly to demand. The only way to keep your production where it was is to pump. You can pump during your lunch break or on other small breaks. I used the Lansinoh Double Electric Breast Pump with my twin boys and it worked great.
However, your production should keep up with demand, so as long as you are still nursing your son once a day or so, ( supplementing the rest with formula) your milk production should only decrease, not stop.
I started supplementing my twins around 8 mo, and noticed a decrease in the milk production, but it didn't stop until I completely weaned them.
Hope that helps!