Milk with cocoa

So LO is al ost 13 months hates cows milk and don’t drink formula.I tried putting vanilla extract he didn’t want it.I made oatmeal with milk nope.I made same dish with water he ate it.

They suggested couple friends to put cocoa in milk or to put sugar .Not sure should he drink milk with cocoa but sugar in honestly against it he is still young for sugar and he gets stomachache when he eats something sweet (cake on his b day he got bad stomachache that he was crying for two hours).

So is cocoa okay or do you have any recommendations?

I tried to give him almond milk but no.

•Edit

•Thank you all mommas for advice. He eats sour cream,yogurt,cottage cheese and I never forced him to drink milk.But my in-laws,my mom or whoever ask about that they are surprised and telling me to force him or add cocoa sugar etc. Like I said I would never forced him to drink that. He drinks lots of water and his pedi is okay with it.