Not sure that having him pee before time-out wuold be the best idea as putting that amount of time delay between whatever actions led to the time-out and the actual time-out may lead him to not even remember why is in time-out. Also, he may start acting out just for the extra attention involved in all that process. I think maybe just having him sit on a waterproof time-out mat would be better and then if he pees, don't say anything about it...give him zero attention for it and just give him dry clothes to change into...if he can do it himself all the better, otherwise help him without a word about it. I think often kids do these things (assuming he has control of his bladder and is doing it on purpose) because it gets a reaction and he can see it gets you riled up.