K.,
He may not be developmentally there. Don't blame yourself. Don't blame him. Just work with it. My son used a cup very early - but he wasn't consistent. Neither was I. I also wasn't concerned about getting him off the 'bottle' until 2.5 when it (according do some sources but not others) has the chance of causing dental issues (what the real deal is, I don't know but I cut it out just before his last molars finished coming in - which is when I figure stuff is done moving around in his mouth).
A cup takes a little finesse. There's a little story about 'Big Anthony' and while the whole story isn't about a cup, a little part is, and I can't help thinking 'they blamed the little boy for not 'paying attention' at what appeared to be a young age.'
Your little guy may not have the eye-hand-mouth coordination to do it yet.
Just keep trying, and maybe try sitting next to him, so he can see the liquid in your glass as it gets closer to your mouth.
I talk to my son through most if not all the new things we try, and I break things down step by step for him. Usually he gets it. : )
Lift up your cup,
touch your bottom lip with this part (point)
set it in your lip,
lift it until you see some water coming in
enjoy!
Just an example. He'll get it.
It's like feeding a child by hand - some cultures do this for a long time - and pediatricians here get antsy because they want 'certain milestones' to be hit on target or whatever. The child will not go hungry because he's been hand fed and can't figure out how to use a fork and knife. Most babies will eventually use a cup. It'll just take a bit.
If he's tossing the sippy though, you might consider one that (if you haven't already) doesn't involve suction to get the liquid out.
Good luck!
M.