This is normal / not unusual. My oldest was fastidious and did not want to mess in his pants; the youngest could care less. With the youngest, I was more relaxed, so I would just put pull ups on him early on when we went out. Note: I was not in any hurry to train him and this can backfire or set you back -- child potty trained at home, not when you go out. Also try scheduling pottying when you're out -- go before you leave, when you get to your destination, every hour, etc. Another thing that worked with us after bare butt for a while was to wear sweatpants w/ no underwear (we were potty training in the winter); maybe loose fitting boxers under the dress will work the same?
Good luck and keep your patience -- it is just a phase of training and you'll all get through it! :)