If your going to do it you gotta go cold turkey - underwear only. my daughter did pretty well with the training, but i was worried about going out with her and public bathrooms...so we'd do a pull up when we went out.... and then she regressed b/c of the pullup. so we just had to go cold turkey one weekend - stay home as much as possible for a weekend, let her run around in underwear and go. you know he can do it since she has done it occaisionally.
Going out of the house was more my issue than hers, and i was scared to do it. put a kit together for the car: a potty, wipes, extra underwear, pants/shorts, sock, grocery store bags to put wet clothes in. OneStepAhead.com has a great folding travel potty that i have and it's great. (embarrassed to say but i've used it before to, when i was in a pinch and had to go really bad) i also made the same kit and put it in a "tub" for outside so we don't have to come back in to go.
but get in the habit of making her go to the potty before you leave the house. then if your out running errands for more than an hour, pull out the potty and make her go. i have an SUV so i just open the back hatch and set it up and she went... i've also done it in a parking lot just put the potty on the ground between 2 cars.
Reward- find something that is small that will work for her. for us it was M&M's everytime she went on the potty she got 3 M&M's. we did this for about 6 mos. then when the bag of M&M's were gone, that was it. don't give her the option to say "no thanks" when you ask if she has to go. make it clear that until she goes potty on his own, she needs to go when you tell him. set a timer so she hears and audible reminder of when to go - so it's not just when you tell her. (i use the timer for everything, when i want them to get dressed, when it's time to leave the park.) since it's a "neutral" reminder my kids don't give me any pushback when the timer goes off. they know it's not negotiable. so i have a small digital timer at home, in mypurse/diaperbag, car etc.....
my daughter was so active she didn't want to take the time.... so put the potty in the LR/playroom if needed so she doesn't have to stray from playing so much (just to get started). then once she gets the hang of it you can put it back in the bathroom. and i would make her say the ABC's if she got through the whole song with out peeing - i'd say thanks for trying and she'd get 1 M&M. 90% of the time she's starts peeing by the time she say "ABCD"
the other part is "liquid management" - when your home for the first weekend of training - flood her with liquids so he has to go alot and get the hang of it. once she's trained... try not to give her liquid 30-45 minutes before you want to leave the house... try not to give her endless sippy cups while you're out to cut down on the potty time while you're running errands. hope this helps
my kids are 23 mos apart.... so my son was about 1 when we started. although he's not trained (we'll work on that next summer before preschool). he's in the habit of when she goes potty, he comes with us, and he sits on the potty too, washes his hands etc..... so it can only help to include your younger one in the training as well