S.C.
S.,
Go straight to the underwear! If he is already going in the potty he recognizes that he has to go. Buy him some with characters that he loves (i.e., Thomas, Power Ranges, Mickey Mouse, etc.) and make a big deal about him wearing the big boy underwear. Remind him that he can't peepee in them and tell him how they differ from a diaper - peepee will run down his leg, etc. If he does peepee in them, he will feel how yucky it is and will probably not do it again. You can also add that "Thomas (or who ever)" will be upset if you peepee on him (only use this tactic if necessary). As for night time, use pull-ups and explain that they are for big boys as well, but only for night time - just in case.
I had a daycare teacher tell me once, when my son was 2 1/2, no to be so selfish. At first I didn't know what she was saying. She said that parents keep their kids in diapers/pull-ups because it is convenient for us. We don't have to worry about accidents and changing clothes or making a mess out somewhere. But all this does is hender the child from learning what they should already know how to do at that age. From that day forward I put my son in underwear and he was trained within 2 weeks.
#2 is a different story - I am still working in this with my youngest - 2 1/2. I don't have any real good advise for you here except to be patient and know that it can be very tramatizing for them, but they will over come it! My oldest did very quickly, but youngest is a little more stubborn.
Good luck!
Sam