What types of sippys have you tried? I struggled to get DD from breast to bottle when she was 6.5months old. Then I struggled with getting her to sippy. I started with a Nuby which is quite "bottle like":
Cups and Spouts < Nuby Products | Nûby?
Then after maybe a couple of weeks to a month, I switched her to a sippy.
Does he drink water from a sippy? If so, are you using the same sippy for milk? If that is the case, try a different type of cup for his milk and keep the water cup only for water.
I still do this. Kirsty will drink water from any cup at all, but she will only drink her milk from 2 specific cups we have. I have 4 cups in total.
When do you offer the milk in a cup? Do you sit it down alongside a meal? Say for example, breakfast... does he perhaps nibble on toast? Maybe sit a cup beside him, no fuss, not verbal confirmation of "here's your milk"... just lay it down and make no deal of it? He may surprise you and just pick it up without realising himself!