Yes - melatonin is pretty tightly regulated i the U.K.
Like another poster on here, I also bought it from BIOVEA. However, for me, it wasn't the magic solution I was looking for. Yes it made me a bit drowsy, but I'm also very sensitive to alcohol and it was unclear if I felt a bit drowsier due to the melatonin, or that.
Be careful with melatonin produced in the U.S. I've seen reports of bottles containing almost next to nothing compared to what's meant to be inside, and other bottles containing 3x or more the stated amount!! One only needs a tiny amount of this hormone, so overdosing might be a real risk factor.
It's the last thing your son will want to do right now, but how often does he go outside and let direct sunshine into his eyes? Not looking at the sun obviously, but being outside preferably on 4 hour long stints?
As an experiment, I would take him on one of those hikes if you can. Please don't tell him why you're doing it otherwise the experiment won't work, but I'd like to know what he'd be like if he got a 4 hr long hike on the weekend. It doesn't have to be very strenuous. The point is not to push him to that extent, but it should be mild-moderate consistent exercise, and preferably in landscape he likes (some people much prefer coast to woodlands for example).