I can only imagine

Coffee although not bad generally is probably worse for a kid than chocolate milk in a different way (caffeine vs sugar causing weight gain)

Coffee is very addictive and for you to give him coffee at that young if and age and possibly get him hooked, is not good for him.
Now wtf is wrong with chocolate milk

Although there's nothing wrong with it itself it might not be a good idea to give it to a child getting ready for bed.

coffee is no more addictive than sugar.

Sugar is actually more addictive.