I'd wait, OP. Your opening post is familiar to me and second post identical to my situation with my daughter. For the past few months she's been aware of when she's going and announcing it, but kicking right off at the mere suggestion of sitting on it. I thought to hell with everyone saying she's ready; she clearly isn't.
The past few weeks she's been wearing pullups - I bought some by mistake a while back, ran out of nappies and used a pullup instead, she loved it. I stopped putting her in vests at the same time, so she's spent a lot of time figuring out how to get out of her trousers so she can, ahem, have a good rummage over these few weeks.
This morning she turned to me and announced: "I need a wee!" I asked if she'd like to do it in the potty. "Oh! Yes! The potty!" she said, and pulled her trousers and pullup down. Then she sat on the potty and stayed there for a good five minutes while staring at the telly. "Aww," she said at last, "it's gone away." I made a big fuss of her anyway. Later we had the same again, but still no wee produced. I left her to it while I went to have a crafty cig sort the washing out. A minute later: "Mummy, mummy! A wee! I did a wee!"
Sorry that got a bit lengthy, OP, I'm just still grinning like a maniac. Wait a bit. If he doesn't want to sit, you can't force him. He'll sit when he's ready to. Things I think may have helped us get here: talking frequently about when she uses the potty like a big girl, not if; introducing a sticker chart to teach her the benefits of doing as she's told (because refusing to sit when told means it's not really going to work); and seeing kids at nursery with potty training reward charts.