Try sugar free "enhancers" like the blue raspberry flavor squeezesby Crystal Lite. That way you can control how much flavor is being added.

Ask your son if he would like to pick a new water container he might like to drink from. Ask him if he wants to mark on it show everyone knows it is his. Ask him if he would like to try a straw from time to time.
Ask him if he would like to put the water in the container himself. Place it on towel and don'w worry about spill. As HIM WHAT CAN HE DO TO MAKE WATER MORE FUN TO DRINK ....SHOW HIM ANIMALS DRINKING WATER. SHOW HIM HOW WATER FREEZES AND ADD SMALL CHOPPED ICE TO HIS WATER. MAKE IT CURIOUS AND IN HIS CONTROL. HELP HIM KNOW THERE IS MORE THAN ONE WAY TO DRINK AND LIKE WATER.

I have demonstrate that I like drinking water. It works on my son but my ASD daughter only drinks water when she gets fairly thirsty and I don't tell what it is.

You could offer something highly desired as a reward for taking a drink. When he takes a sip, make a big deal of it (cheers, etc.) and give the reward (candy, 2 minutes of a favorite video). Do it repeatedly throughout the day (so a non-edible reward would be best). One drink = reward. After that is mastered, up the ante to two drinks.

He will likely tantrum at first. We just say, "OK, let me know when you're ready." If you withhold the desired reward for ONLY that activity--he will likely come around.

Meanwhile, you can still give him juice/water so he's getting enough -- but I like the idea of having him drink it consciously in the end.

I would continue to add the juice and reduce the amount until it is almost straight water.