He sounds normal to me. Potty training is sometimes 1 step forward and 2 steps back.
He's interested, and that's great! Don't squash his interest.
He is just learning how to control his body, and what it feels like to have to go potty. Hang in there. You are right on by taking him to the toilet every so often, and having him in underwear. Like someone else mentioned, don't ask him, just tell him it's time to go. Set a timer if it helps. Take him to the bathroom every 20-30 minutes if you have to. When he sits on the potty, even if he doesn't go, give him a treat: a jelly bean, a sticker, whatever.
Sometimes kids feel singled out when mom is constantly bringing them to the bathroom. If it helps, you and your daughter "play along" and make it a family potty break. Kids usually want to do whatever their older siblings are doing. When it's potty break time, have your daughter go in the bathroom first, even if all she does is just close the door, count to 20, and flush.
Lastly, if he isn't trained by the time preschool starts, he's simply not ready for preschool. It won't be the end of the world!