Please do NOT beat your self up over this! This is really common. I started supplementing with formula with both my babies around 9-10 months. We still always nursed when I was home until they self-weaned (one at 12 months, one at 16 months). The trick, as Nicole mentioned, is to start before you run out of breastmilk in the freezer, so that you can mix them to get him used to the taste. Start by just topping off the bottles with formula (I used to do 3 oz breastmilk, 1 oz formula), then, as your supply slows down and your freezer runs out, gradually increase the proportion that is formula in each bottle.
Also, for what it's worth, with both of mine, like you I was SO TIRED OF PUMPING at work, that around 10 months, I actually cut out one pumping session a day (for me, that was from 2X to 1X). It did make my supply drop more, leading to more formula at daycare. But, both babies were happy to keep nursing when I was home and it helped me to keep my sanity.
Don't stress, you are doing a fabulous job!