Hi A.,
I went thru the same with my 2 boys! My first was done in a few days, easy as pie, at 2 years 9 months. Then with my second little boy, I tried a few months before his third birthday because he was showing all of the "classic" readiness signs for potty training. Well, he didn't care, 6 accidents a day in his undies, wouldn't tell me when he was wet. I too tried all the motivators: stickers, M&Ms, verbal praise/hugs when he did go, but it was RARE! I gave up after about a week and stuck him back in diapers. For a while we didn't talk about it at all. Then occasionally I would mention the potty and if he wanted to try to use it and wear big boy underwear, and he would do the same as your son, say he didn't want to, that he only wears diapers.
Well, his 3rd birthday came and went in January, and I was feeling discouraged, because I didn't know what to do, and felt like he was "too old" to be in diapers, especially since his older brother had already been out of diapers for a few months at this age, but I really didn't talk about it too much with him, or push it too much with him, because I didn't want to create a power struggle. I wanted to try and act like it didn't bother me and I didn't care about it WITH HIM....sometimes I feel like my boys will do the opposite of what I want.
Anyway, at about 3 years and 2 months in March, he just suddenly decided he wanted to wear underwear! and that was it! He WANTED to do it, and he DID it, in a few days! There is a lot to say for self motivation!!! Once he decided he wanted to do it, and wasn't doing it because I was asking him too, he was SO much more willing to try, sit on the pot, was excited about the prizes I got him (matchbox cars-because they were special "potty cars") and would tell me if he was wet, because I would remind him to keep his underwear dry. Also, the timer worked great with my first son, not with the second son. He would tell me he didn't have to go even if he was holding himself because the timer didn't beep yet.....so I had to get rid of it!
Anyway, I know you have the preschool factor, mine will be going as well this Sept. for the first time, and if you can, I would just hang it up and wait a bit, to see if he wants to do it on his own. You still have 2 months, and a lot can change. It may be kind of last min. in August, but, he won't be the only one just learning. Also, I would ask your preschool now, if he's not ready in time, will they hold his place for a month (October) before they fill the spot with someone from the waitlist? That may buy you another month, w/o losing his spot.
Good luck! I know your stressed about it, but try not to be, my husband was as well, and I keep telling him he wasn't going to go to high school in diapers and he needed to calm down and just let him do it at his own pace. Good luck!
~A.