Starting transition to cows milk - 11 Months

Today my son is 11 months and 11 days and I made it the farthest I could with only giving him breastmilk. My freezer stash is gone and I only produce about 7 ounces at work and he drinks about 15 ounces during that time period. So today's the day he starts drinking some cows milk and I will slowly transition him to all cows milk. I just hope the process isn't too painful and he takes the milk, it will be mixed with his breastmilk. I can't believe I'm already at this point.