Not quite sure why you'd wean him from human milk to another mammal's milk... but, yes, it's far too early for straight cow milk. Their milk is extremely high in calcium and sodium, so much so that it's extremely dangerous for human baby kidneys.
You can know that he'll be okay in 3 months because by they eating milk for three days --or not-- is no big deal. He has no reason to be drinking cow's milk at all (it's ONE healthy food choice for those who like it and want to, but hardly what the Diary Bureau would like you to believe: a 'necessary' human food)... and whether or not he drinks anything out of a sippy is irrelevant. He can get all the liquids from the other foods (soups, fruits, vegetables) without drinking anything else.
If you feel the need to leave milk, try the manual expression technique: most women find it far more effective than pumping. Google 'Chele Marmet Manual Expression Technique' for instructions. If you want to bother. There's really no reason to.