Find a toy such as one that has buttons to push and is really exciting. Tell him he can play with it for 5 minutes after he goes potty in the toilet. Then keep a chart and take him every half hour (the first day) giving him the toy only when he goes in the toilet. Then when you know approximately when he usually voids, expand the time between going to the potty...except if he asks to go...then by all means go. Only when he goes in the toilet, give him the toy for exactly 5 minutes, then it goes away until he goes again. The toy usually continues to be novel until they are trained. The toy is his when he starts asking and going on a regular basis.
I was a special education teacher and had a 4 1/2 year old how was not trained and with home and school cooperation, we managed to have the little guy trained in about 2 weeks. His mother had just about decided he was not going to train because his younger brother, just over 3, was already trained. Mom was so delighted! With a normal child the process should go faster:)
BTW, he is not that late. I had three sons and none of them were trained until after 3 years...was sometime in the third year they trained. Since it is summer, do let him go without diapers or the disposable pull ups that absorb and they can't even tell they have gone. Unfortunately, my one son wanted his off in the winter and his pants, shirts, socks and shoes would all get wet!! What a mess...I finally had to post pone him longer until summer when they played in the yard most of the time. Just remember, don't scold or put pressure on him when you take him. Just take him on a regular basis. When it clicks he should be just about completely trained. They all have some accidents after training:)
Just read some of the other responses...the ones with matchbox cars on the back of the toilet might work, but is such a big temptation, while one special toy he really wants (transformer..or is he too little for that?) you can put away so he can't get into it by himself. We used a Pooh Bear that had fruit all over his tummy to push and Pooh would laugh and name the fruit).
Hope that helps you,
H.