Here's the thing, Dad... If you recall when you were your sons' age, it seemed as if your cock never went soft, and your balls ached all the time no matter how many times a day you stroked one out. You wondered if you were some kind of pervert, addicted to playing with your pecker.
If there was no information coming from your father, no older brother you could confide in, what did you do? Did you have a buddy or two you could ask, "Is your dick hard all the time? Do you...you know...jack off?"
Your boys need to know they are on common ground here. 98% of the male population wakes up with that log of wood your son was so freaked out that you'd seen. You need to tell that youngest son that not only is it normal - it's a damn good thing he gets them.
You need to tell both of your sons in that sit-down, that it's common for boners to hit us so many times a day that, at some point in our pubescent years, we really did think they never went down. The jokes about hiding our stiff cock bulges behind books didn't come out of nowhere. And, the phrase "Perpetual hard-on" exists for a reason.
As far as wacking the willy, we all did. And, barring some medical issue or other, we still do.
We don't confine ourselves to jacking off while hiding in the bathroom, either. If a boner strikes and won't ease, we'll find a way to relieve it no matter where we are. Particularly if it's extremely hard.
Don't make it seem as if you no longer masturbate, either. Let them know you're human. That you still enjoy pleasuring yourself. And, with any amount of luck, as we military guys used to joke, "I want to die with my hard cock in one hand, and a beer in the other."
I applaud you for being such a cool dad. Your sons' friends will wish their own fathers were as open and honest. If they begin asking you for information
be the Koolaid Dad - step up to the plate and give them honest answers.