My son is 9 and I have dealt with issues also of speech delay and also had trouble with potty training.  I know all kids learn and work differently, but the reward process worked well for me.  Every time he would go to the potty we had a chart and first he got to put up a sticker and then I would give him a small reward / treat (usually a favorite food or activity) that he really liked.  After he would earn so many stickers then he got an even bigger reward (sometimes a small toy, sometimes a trip out to his favorite restaraunt).  He loved earning the rewards and it was very effective.   Good luck!!