OK. What did our parents do...they went straight to glasses. Yes, glasses. If you son is really strong willed, he believes he can do anything (like my son!). Tell him he is right, sippy cups are for babies and he is a big boy and should use a regular glass like everyone else. Yes there will be spills - some of them on purpose, but give him a towel and let him clean it up. There will be broken cups at which point you have him watch and help you (by holding trash can, getting broom, safe stuff) while you clean it up and then get him another real glass. Plastic cups will tip over much easier than glass and after having to clean up his own messes enough times he will get the idea to not spill!
There is a wonderful company called Montessori Services that sells child size juice glasses and even pitchers. You would be absolutely amazed at how much kids LOVE to pour their own drinks and use glasses. It is very liberating for them and a huge confidence builder in a very loving practical way.
Good luck.