It sounds like he's doing really well! He's not too young, i potty trained my son at 20mths. If he's telling you he needs a wee and doing it in the potty then that is excellent. Give him lots of praise and you could give him a little treat /prize every time he does it. I would keep the pants on and don't have him running around with nothing on for the time being. just give him little reminders that he must tell you when he needs it, but don't ask too often. If he wets i agree to keep his pants on for a bit and when you change him just say, never mind next time you'll get it in the potty. Try not to get annoyed and don't mention that he didn't tell you. If he does tell you and does go in potty then again praise him and give him a treat/prize.
I wouldn't worry too much as it sounds like you've done the hard bit and i'm sure it'll be all ok very soon.