Good to hear, and that tends to be how it plays out. A black box policy is basically the insurer saying "we can't price this driver yet, so show us", and after a clean year you've got real data on your side. Premiums for new drivers usually fall noticeably after the first year anyway, and the telematics record gives you extra leverage on top of that.
A few things worth doing before the renewal goes through:
Get quotes from a few places rather than just accepting the renewal price. Insurers often loosen up on loyalty for telematics customers, but the best deal often isn't with the current provider. Check whether you can keep the black box if the price is better, or move to a standard policy now that he has a year behind him. Some insurers will give a better headline price for a standard policy after 12 months of clean driving, but it's worth comparing the two directly.
Check what the policy says about his driving score. Some black box policies reward consistent scores with a lower renewal, so if his driving has been smooth (no harsh braking, no late-night driving if that's penalised), ask the insurer whether that's reflected in the renewal price, because it isn't always automatic.
Keep his no claims discount protected. A year of no claims is worth a lot at this age, and protecting it is usually cheap.
Be careful with the details when you re-quote. Postcode, where the car is parked overnight, annual mileage and whether you name yourself as the main driver all move the price a lot, and the details have to be accurate or a claim could be affected.
If he's thinking about changing car at some point, it's worth checking the insurance group and getting a quote on the specific car before buying. The difference between two similar-looking cars can be a few hundred pounds a year at his age.
(I work on a UK first car tool, FirstCarScout, which is partly why I'm looking at this stuff so much, but the above is just what tends to work.)