Not really precisely on point, but I have tried always to talk to my kids about money during their entire lives (they're now 27, 22, and 20). I've tried to be honest and accurate, of course. I've also tried to start with simpler things and build to more complex things, and more general comments to more specific ones. I've tried to move the conversation forward by sharing more information as time goes by.
But I've also tried to do a just in time approach, where I share information just a bit before they need to know it. I find their interest level and comprehension level rise greatly when I can connect it to something that's relevant to them.
I also try to share values and lessons along with the facts.
So in your situation, I would first make my kid aware of the 529 and the UTMA, and that the money in the 529 is for college and the UTMA is for college if needed then could be used for a house downpayment or a wedding or a car or whatever it is your family values are and your intentions were.
Then at some point in the next few years I'd probably sit down and show him actual statements with account values and tell him about the age 21 thing, and show how much the college costs are so he can understand the math and how much would be left over. How soon before age 21 sort of depends on the kid and his money personality, but I'd probably at least do a year or two so he has a chance to absorb the news and think through all of his options and allow for time for you and him to have conversations about what to do with the UTMA money.
Insisting on POA is an interesting thought. I have been going the opposite direction with my kids lately. I used to fear them making mistakes with their money and thought I knew more, so I tried maintaining control. Then I realized a few things. First, everyone makes mistakes, especially starting out in a new area. Second, making mistakes early is how they learn. Third, making mistakes early usually involves smaller dollar amounts which are usually not fatal to long term financial health. Fourth, making mistakes while I'm still around and having it be an open topic means I have the opportunity to share my values and discuss with them and learn about their values (which sometimes trump what I think is best to do in some financial situations).
So now my kids are making more and bigger financial decisions on their own. Nearly all the time they do very well. Sometimes they make minor errors, but usually only once and then they course correct because it's their mistake, not mine. Very occasionally I'll disagree with their intended course of action, but they are adults and are the ones who will have to live with the consequences, not me. It also helps turn our relationship into an adult-to-adult relationship rather than parent-child, which is intimidating at first but awesomely wonderful as time goes by.