Attached to his cup at 16 months?

It is okay for kids at that to still be using a "sippy cup" or "straw cup" but not for the comfort. His teeth actually can be damaged as far as the enamel because of any residual milk and or juice staying in his mouth or by continuously drinking. I realize you did say that he doesnt always drink the liquids in the cup but if there is any in there-he can. The teeth also can be damaged by the sucking just like with a pacifier. Maybe even more so because it is a harder more rigid material. I think the offering it only when he is thirsty and saying "cups are for drinking" is a giid idea. I would also recommend offering a regular cup with just a small amount of juice or milk at a time. Yes i know it can be messy but it doesnt take long for them to get the hang of it. Either that or take it away "cold-turkey" good luck