Hi, well done for breastfeeding so long!
Will he take expressed milk in the sippy cup? I would perhaps try a cup of ebm perhaps at a different time of the day so that he gets used to it, and then transition onto him taking this for his morning feed. You might find it helpful if someone else was to to this. Then you can mix the ebm with cows milk, slowly eventually to it being just cows milk. Isn't such a big shock for him then.
Also L really liked to take her milk out of a beaker with a straw, it gave her the sucking comfort she lost with breastfeeding.