Hi O.,
The hardest part about helping children use the toilet (at least, from my experience working with young children) is to help them figure out how to buy into *why* staying dry is important for them.
Because you have a little guy who you say is 'stubborn', he may likely experience having to hand that token over to you as a punishment. It may feel like you are upping the ante for him because you are not focusing on his success, but are actually making the focus of attention about his failure to stay dry.
If you want to do a reward, I would do it for staying dry ALL day. If you feel that he can control his bladder/bowels and that pants messing is a conscious choice, then I'd have a fun little something for every full day that he stays dry.
Be aware that, when using external motivators like rewards, there is sometimes the complication that,when removing the motivator (as it were), the child regresses to the behaviors they were exhibiting before. So, know that you may need to be committed to offering the reward for some time. In which case, you do want to keep it something inexpensive and easy to stock up on.
Another method some parents use is to find a 'currency' item (an activity or toy your child values, but not a favorite security toy) and to put that away/not offer it until he 'earns' time with the toy/activity by being dry all day. Then, the child can either be allowed to have that time (15-20 minutes) that evening or you can give him little 'tickets' or certificates for the time you will owe him in the next day or so with that item.
When my son was about 3, we did try using the potty-- he simply wasn't interested. I reintroduced it about four months later, simply telling him that he could either wear cloth diapers or underwear, his choice, but that the teachers at school said he was old enough not to need paper diapers during the daytime. He was motivated by *hating* the feeling of wetting himself, so that was simple. I still think the best motivation for kids is the feeling of staying dry; we point out "wow, you got to the potty so fast, we don't have to spend time changing wet clothes. Good for you!" Simple, clear feedback is helpful too.
Maybe something here will work for you... good luck!