agreed on the underwear and no pull-ups or diapers! Keep dressing him in comfortable (think cotton, elastic) shorts/pants. You can even let him go commando underneath if mommy is okay with it. That way there are no frustrating clothes to deter him. Letting him pick his underwear each day can be fun too ("do you want to wear the cars or dinosaurs today?" with a big enthusiastic smile)
Make it a rule/routine that he (or everyone) goes potty at regular times, such as when you wake up or after breakfast, before getting in the car, before nap, before bathtime...
We keep a small toybox in the bathroom, and that helped motivate our lil' to sit on the toilet b/c she could play with a small toy or read.
You could also do the stickers reward chart