My boys are in their teens, but your post brings back memories. The first thing I would say is don't get too stressed over the issue - it will happen and your son will eventually move to the toilet. It may not happen as soon as you expect, and it may not go as smooth as the soother, but it will happen. Change can be very, very difficult for some little kids. Actually, change can be very difficult for some adults too - I know how resistant I am to change at 50 years old, so it isn't surprising that the little kids have problems with change as well.
One of my sons in particular was later than every other kid at everything -- soother, diapers, talking, swimming, reading, writing, etc., and I spent years worrying about him. I'm embarrassed to say it, but I think he was 4 before I got him off of diapers. Once in school, there were a few years that I was sure I should send him back a grade or two in school because he was so behind everyone. Now in high school, he is one of the school's top students. But for years, he was by far, the slowest student at absolutely everything. There are many mothers who comment that their boys are later at achieving certain milestones than other children.
One of my friends had a similar problem with her son and diapers. She chose a warm weekend and spent it outside with the kids in the yard playing. She had her little boy play naked outside all day, and she brought the portable toilet outside to try to coax him to use it. And she firmly disallowed diapers outside, no exceptions. I guess that somehow he eventually made the connection that without a diaper, he might as well use the toilet.
Have you spoken with your doctor - maybe he/she could offer some advice?
For me, I had tried patiently for 2 years to move my son off the diaper, with no luck. I also had tried all of the advice from books, doctors, etc. You'll probably not like my method, but I was absolutely desperate - my son was 4! I took myself, and my 4-year old son, and my 2-year old, and we went into the bathroom! We spent the waking hours of the day, for 3 days, in the bathroom!! and I read to them all day long. No diapers allowed. Of course, I fed them, and made the day as relatively enjoyable as I could by reading to them. My 4-year old son eventually had to go pee, and I firmly stated that he wasn't getting a diaper. He could use the toilet, or pee on the floor, or in the bathtub, or whereever, but no diaper! He was very, very distressed, crying, etc. He held his pee for about two hours because he wanted the diaper. Eventually he peed all over the floor and wall. Without getting visibly upset, I just calmly cleaned up the mess. We went on reading. This happened over and over and over throughout the day. Probably by the end of day 2 or early day 3, he eventually, reluctantly tried the toilet. Of course, I made a really big deal congratulating him for it. I think that by the end of day 3, he could use the toilet fairly reliably.
It took me 3 days to essentially force my son off of diapers, and I was so upset by the late evening on each of the days, that I would cry and cry. But all of the advice from books, doctors, friends, to try to coax him into using the toilet didn't seem to get anywhere for me. And I will also say that bribing him to get him to do anything never worked then, and doesn't work now. There is no bribe that he wants enough to make him do what he is uncomfortable doing.
In any case, good luck.