Your son is old enough that this has become a will issue. So you need to avoid frontal confrontations and go the sneaky route. First of all, let him run around the house buck naked. No kid likes to wet down his leg. Be prepared for some messes. Make him stay in the same room as you so when he does start to go, you can rush him to the toilet. DO NOT SCREAM/YELL/BE ANGRY when this happens. (I do not know your child; while assuming because of his age that he is being willful, it could be that he just doesn't get it. So grabbing him and saying, "Wait! Wait! Potty on the toilet!" and running and putting him on to finish might be what he needs for that lightbulb moment.) Anyway, only let him have pants on when he is sleeping or you are going out.
When he does have an accident, make it unpleasant in the nicest way. (This is what I mean by not being angry or confrontational.) "I'm so sorry you had an accident! That is so sad!" (pick him up.) "We'd better get you cleaned up!" (take him to the bathtub and turn on freezing cold water.)"I know this is cold but I'll be quick! We have to get you clean when you have an accident and don't go in the potty!" (Make sure you splash cold water all over him in the most loving way while he shrieks and you reassure him in your most loving voice.) Take him out, towel dry him and say, "Next time let's go in the potty so we don't have to wash!" Do this EVERY TIME he has an accident. This reinforces the idea that the accident is more upsetting/painful then the toilet.
Put him on the toilet consistently every thirty minutes. Set a timer, from the time you come home until his bedtime. And he does not get any toys or candy while he is sitting there. He gets the candy when he has been successful. If he sits there and does nothing, you say, "Maybe you can have a lollipop next time when you potty big." This is not an option. You do not ask. The timer goes off and you swoop him up and say, "Potty time!" and put him on. Once he realizes that he cannot get out of this even if he has a fit, then you can make it a game. Timer goes off, you yell potty time! Run! and the two of you race to the toilet. When he is on the toilet, you do not interact with him much. You clean the sink, organize a shelf, whatever. Do not leave him on for long...no marathons here. Just a few minutes. Turn on the sink and let the water run because sometimes the sound of water will help him release those muscles and go. The minute he starts going, Whoop and holler and praise like crazy. Have him go tell his dad and call his grandma. Get people to come look. These steps are important to his WANTING to go potty. After he gets in a routine of successes, then start asking for more. He goes a minute, looks up at you expectantly and you say, "Go more!" Sometimes that helps. If he actually poops on the toilet then you have to do something very special. And you definitely have to share that information with the world.
And lastly, remove every sweet from his life. No sugary drinks, no candy or desserts of any type. Those are reserved only for successful potty time. That is the motivator. Why would I go potty if I've already gotten the reward anytime I screamed for it? Make it visible but unaccessable. Say, "You can have some when you potty in the toilet. And be very consistent on this. Let him watch you go potty and say, "Now I've earned an (M&M or whatever) and then go eat it in front of him. If he wants some say, "You have to go potty! Let's go! Then you can have an M&M!"
I am actually in the middle of potty training a 2 year old boy right now...my 5th to potty train. I don't reward a drip or two, only a real stream. And we still have accidents but he is young still. The only way to make progress is to be SUPER consistent. It has to be a total priority and it would help if you could get your daycare on board with the timer and taking him every 30 minutes. It would also help if you could get them to agree to the no sugars/sweets.
Because he is older and has refused so far, it will be tough at the beginning. But if you are tough, loving, and consistent, you will see a change in less then a week. He will quit having fits every time you take him to the toilet when the timer goes off. He will quit screaming for candy that he sees and cannot have. So hang tough for the first week and be uber consistent and half the battle will be won.
Good luck.
VickiS
Just an additional note...right after I wrote this I was taking my usually very compliant 2 year old to the toilet...I said, "Time to go potty!" and he said, "NO!!" and laid on the floor. I said, "I'm gonna beat you! I'm gonna beat you there!" and started singing a "I'm going to win" song and I started moving my arms like I was going really fast but I was taking tiny half steps. He got up and ran and beat me to the toilet and went potty. Presentation does a LOT.