FWIW I'd be looking at used hybrids and either biting the bullet to inspect / maintain it yourself or trying to find someone independent locally to do it for you.
Unfortunately bikes are sodding expensive and money is evidently a significant concern (as it is for many of us). You mention £300 as being pricey but tbh given today's market I'm surprised you can even get anything worth having for that money new.
Bottom line is something decent is going to cost you and he's likely to grow out of it fairly quickly. Used bikes are typically a lot cheaper than new and the market's seeing a bit of a slump currently..
My money would be on something unpretentious and basic - a used hybrid of some description; probably ally as all the budget stuff is. See how he gets on with it, use it as a learning experience and sell it on for close to what you paid for it if he grows out of it / wants something better / doesn't get on with it.
I reckon the sort of stuff you're looking at new for £300 could probably be had for less than a ton if you keep an eye on the usual places.
A few after a quick trawl on FB muppetplace:
Apollo Envoy for £80
Ridgeback velocity for £100
A decent looking Carrera for £100
Giant CRS for £100
A no-name, matt-black roadi-ish offering with budget Stis for £80
Specialized Vita for £150 (maybe ladies but not obviously so)
A Boardman Team for £220