My now 14 year old son went through that phase, at about the same age, using that exact word. I spoke to my pediatrician about it because it was getting out of hand. Since it had gone on for a few weeks, and my son knew by then that he was saying a bad word, he suggested starting with time out each and every time he said it. To my exhaustion, that did not work, alone. However, when my son would say it, we would immediately stop whatever we were doing, pick up all his toys or quit whatever he was playing, put everything away, turn off any T.V.'s or radios and he would stand facing a blank wall for two minutes. My son was a very "spirited" child therefore the quiet, non-stimulating space was very boring for him. I did this for another two days each and every time he said that word and by the third day he did not say it again. I did carry this over as best I could when we were away from home. One time he said it at the grocery store, so I quietly took him out of the cart, we moved to the side of the isle and I had him stand facing the baked beans for two minutes. I could tell he was shocked that I followed through outside of our home but that made a huge impact and I honestly cannot remember having to do a time out with him in public after that episode.
I think the key is consistency. Stick with whatever counsequence you implement each and every time he says that word or anything else your child may do on a regular basis that you want them to stop doing. I am an in-home daycare provider and care for 12 children each day. I had one 5 year old come to daycare and say a naughty word. I explained that his language was inappropriate, I had him pick up all of his toys, come inside and stand at a blank wall for five minutes. He has not said that word again.
Stick with it and I can assure you, it will pass!