Unfortunately, you can't MAKE someone use the potty before they are ready. I don't know how old he is, but my older children didn't really get interested in using the potty until they were at least 2 1/2, but I didn't push it since frankly it is easier to change diapers :-D.
That being said, here are some tricks that worked for us.
First, my kids love the Elmo Potty Time video. I played that over and over again and that got them excited to want to go.
Second, I let them watch us use the potty. And if your older kids wouldn't mind, let him watch them...kids want to be like the big kids!
Third, we got a "potty watch" from Potty Time inc. This is basically a timer they put on their wrist that YOU set to go off in 30, 60, or 90 min. intervals. My kids loved that it lit up and played music when it was time to go and this really moved the process along for them...until I got this, I had a hard time getting them to stop playing with their toys to go. They also loved showing everyone their potty watch. And it helped me not forget to take them and made going peepee a regular thing.
For going poopie, one thing that helped my son was getting him a special book to read while he tries to go poopie. I put him on for 10-15 minutes every day after lunch and dinner with the special book that he only gets to read while he is trying to go poopie. The combination of having something to distract him and doing it at the same time everyday helped to get him to go regularly.
Finally, I let them choose their own "big kid" underwear and have them wear it all day except for bedtime. If they have an accident, I make them wear the generic underwear (the ones that they didn't pick out). They are so proud of the underwear they picked out that they hate having accidents.
And I'm sure you are doing this, but EVERY TIME he goes potty...PRAISE, PRAISE, PRAISE!!! (a sticker or special treat doesn't hurt either! :-D)