To get your 6 year old son off the baby bottle, simply remove the baby bottle and get rid of it.
Then give your 6 year old son a regular cup and while he will likely throw a tantrum for a few days or so, your 6 year old son will eventually give up and start drinking from the cup.
My 6 year old brother years ago was also still on the bottle and by age 7, our mom eventually just took it away.
He threw a major fit and wanted his bottle back, but our mom and dad never gave in and he eventually after a few days started drinking from a cup.
At 6, as long as he has not other issues, he should be drinking from a cup normally and simply taking the bottle away is the best approach.
You could also try cutting the nipple of the bottle a bit at a time, just like some parents do with a pacifier and he may eventually give it up.
Or you might try putting something bitter tasting or even hot like hot sauce on the bottle nipple or add something food safe, not tasty to his drink in his bottle.
Then he would hate drinking it and then offer him the cup of good drink instead.