You said your child uses the toilet every once in a while. In that situation, I would just use diapers. They are cheaper and more absorbant than Pull-Ups. Pull-Ups are made for the ocassional accident and unless you plan on changing it every time he pees, you could have some leaks.
My dd3 had been fully potty trained for months before went last November but I was concerned about her telling me because she was so excited and was afraid she wouldn't want to take breaks to use the potty. I was also concerned about her ability to hold it when we were lines. She did great telling me she had to go -- if she hadn't said anything in a while and we passed a restroom - in we went. We joked that we toured every restroom in all of the parks. We had no accidents from her during the 9 day trip. My son, who was 5 (and had been potty trained since he turned 2) had one accident at the MK -- he waited too long to say anything and then daddy couldn't find the bathroom (we were in Toontown Fair) -- I had to buy new underpants and was planning to throw out the old ones. The cashier knew what had happended and said they see that all the time (even from bigger kids) and gave me a small bag for the soiled ones.