Ditto what everyone says about having him drink a LOT. Make sure it's water and not sugary drinks -- and yes, juice is still sugar if it's 100 percent pure fruit juice. Limit it. Check the color of the urine as others say. If he likes popsicles, make your own out of half water and half juice and have him eat those; they are the same as fluid. You can get popsicle molds all over the place now, even at the grocery store.
My idea to add: He is still young enough to think it's great to get little rewards.Tell him that every time he successfully pees in a day (not just sits but actually produces urine, even a trickle) he will get a bead that he can put in a special jar. When he has four beads in one day, he gets a very tiny reward gift. When he has two whole days of four beads each day, he gets a slightly larger reward. Four whole days of four beads each day, still larger reward. If he gets five beads in any one day -- have something else special for him.
Yes, it is bribery, and it's perfectly fine. He will not always require the rewards in order to pee. If it's too much to keep up with, make it one reward for going two days with peeing four times a day or whatever instead of doing it daily. I would not make him wait a whole week for any reward, though; he is too young to wait that long for the reward and will not connect the reward with the behavior after that long.
The reward can be really tiny! A mini-packet of gummies if that's his thing. Or a tiny notepad and pencil. Or a tiny magnifying glass. You can get things at the local dollar store, and party favors make great little reward gifts.
Also, do you have books or other things he can occupy himself with while he sits on the toilet? He may hate sitting there because he wants to be busy and playing constantly. Have new books that he's never seen before and keep them JUST for toilet time. Really talk up how these are new and he's going to love them but can only have them when he sits on the toilet. You can get a ton of books at any thrift store or yard sale. Put away a bunch of them so you always have a new one to pull out and say "Here's a new book for toilet time!"