Maybe he's feeling too much pressure. Try making it fun and see what happens. Tell him you have no expectations. If he goes fine, if not we'll try later. If he's frightened of the flush, make it a game to flush cheerios down the potty. There are lot's of books on the topic for children. Go to amazon and under children's fiction you can find many funny books about kids afraid of flushing.
Try using a "potty pal". I used a teddy bear, and just before the time would come and I'd think my son would need to use the potty, I'd say "Oh, it's time for Berry-bear to use the potty." Then I'd lead my son and the bear to the potty seat (don't do this in the actual toilet or you might clog it!), and have a couple of miniature candy bars (oh henry's and babe ruth's look convincing) and a tiny squirt bottle of water on hand. I'd take a diaper (used a real one) off the bear and seat it over the potty like he was going to the bathroom and would squirt the water and then drop the candy from behind to give the impression he was going numbers 1 & 2. You can add yellow food color to the water to make it more believeable.
My son thought this was hilarious, but all the while was actually taking it all in. The important part here is to let your son look into the potty bowl and see the "good job" Berry bear did. We'd clap and then we had the bear take tissue, and even pretend to wash his paws. Then I'd ask my son if he was ready to do the same. Sometimes he could go right away. Sometimes not. But I think the routine helped he relax because he enjoyed the stage production so much.
Another thing I'd do is leave some books about potty time in a nearby basket so he could read about going potty while sitting on the potty. I'd tell him it was okay to take a while if he needed to. Sometimes I think he got a little stressed out about "performance" with me hovering nearby, so I gave him a little space and would either sit down nearby but not near him and read too, or straighten up the bathroom and he'd be able to go.
If he's not liking his potty seat for whatever reason, try using the big potty now. My oldest didn't like potty chairs/seats. So we used one of those cushioned rings. When he needed to do #1 we put cheerios in the water and he'd do "target" practice. He couldn't wait. He'd find reasons to have to do target practice. For number 2, we named it "making rocks". He was a big fan of Bob the Builder and construction and so we'd make pretend we were quarry workers making rocks for the crusher. He couldn't wait to send the rocks to the crusher (flushing) and would get the job done pretty quick once we made it this big game.
Just some ideas. Hope they help.