Walking horse that refuses to gait?

My boarder has a 4 year old walker gelding. He will go anywhere, stand for saddle mounting everything, but gait. Not even a try, a slight movement nothing. He sire was shown big lick and the mother was just a trail horse, but at least she gaited. We are at witts end, does anybody know what to do?? We have tried weights in front and he just steps higher and trots on.