Absolutely don't worry about it. At all. Rafael's answer is perfect, but I just want to add some extra context.
I used to commute every morning directly west over a bridge from a barrier island on Florida's east coast, with very few tall buildings on the island to block the morning sun behind me. The bright sunrise was behind me, but annoyingly, it reflected off of two main things:
- the tall office building with mirrored glass windows at the west end of the bridge, effectively putting the sun directly in front of me; and
- the slightly convex rear window of the car in front of me. Because we're both moving at the same speed, once the sun reflected off the window into my eyes, it was like having a mirror mounted directly in front of my car, pointed at my eyes.
Your son is causing neither of these conditions. The building is like a massive mirror, meaning I saw the sun "travel" across it for a long time while it reflected into my eyes.
- Your son's lens's front element, probably about 100mm diameter at most, at 300 ft is actually smaller than the sun's apparent angular diameter to our eyes. Meaning, at 300 ft, your son's lens isn't even reflecting the entire area of the sun.
But more importantly, if his lens did reflect the sun into a driver's eyes, it's only for a vanishingly small moment, as the driver traverses into the narrow reflection path, and then immediately out of it.
And finally, if your son's lens has a lens shade, that will help reduce incidental reflections into other people's eyes even more.
But truthfully, don't worry about it at all. Any reflective glass or polished metal surface larger than your son's lens's front element is a bigger risk of dazzling glare to drivers than his lens is.