My son’s name is Jack and we try to do a costume each year playing on his name (A Jack from the game Jacks and this year a Jack in the box). The first year we did Jack Jack from The Incredibles.
I got red footed pajamas and some fabric paint. I just painted on the Incredibles emblem. I bought some black fabric to make a mask and all I did was cut eye holes and attach a piece of elastic. I then put gel in what little hair he had so he could have the little bit sticking up.
It was a warm costume for the cool fall evenings (We lived in Nevada at the time). It was also comfortable enough to let him sleep in it if he fell asleep while trick or treating.