Breastmilk can be frozn and used up to 6 months later if it is frozen right away and kept in a strong freezer (sounds like yours).
I went through something similar with my son. He was done nursing at 11 months. Just wouldn't sit still for it! He was already drinking well out of a cup (not a sippy cut, just a regular one.) And I thought it was silly to get formula for one month. But the pediatrician said that, while introducing a bit of cow's mile was ok at 11 months, he should still drink formula or breast milk for the main part of his milk intake until he is a full 12 months old. I had been freezing milk all along but I'd used most of it for cereal and when I was at school. So I bought one can of formula and gave him that until he was a year. It wasn't a really big deal.
And I would suggest you start teaching your son to drink from a cup now, if you haven't already (again, not the sippy, just a regular cup). He can learn to drink his milk/formula from a cup by the time you are drying up. And by then, he can also be drinking juice from a cup once a day, as well as water, and eating plenty of solids, so he won't be needing more that about 16-24 oz of milk a day.