Hi there Dee,
I'd recommend expressing just enough to ease the uncomfortable feeling but not enough to make more milk which is what you're doing anyway. Wear a very firm fitting bra (even when sleeping) and try to avoid stimulating your breasts especially in the shower under the water spray. It can be very uncomfortable for at least a few days until your body realizes that you don't have the demand for it.
You can also start adding 10mls of pre-mixed formula into your expressed milk feeds every day or two so that he can slowly adjust to taking formula alone. Also, it's not unusual to still have a little bit of milk left over even when you think it's dryed up, I could still express tiny amounts for 3 months after I stopped bf'ing.
Another great tip to avoid constipation when using full strength formula is to add 10mls of extra water into each bottle (as recommended by my health nurse).
If you've got any queries about formula etc, feel free to pm me :)