Another vote for speaking to unicycle.com or looking on the website. Good range of budget models. There’s also a few clubs dotted around the country. It’s pretty easy to learn just takes a bit of commitment and not minding falling off. Maybe a couple of weeks of 20 mins practice sessions.
A 20″ freestyle is the standard for learning on but it might be worth going something a bit more trials orientated if he has an interest there. It’ll be a little harder to learn on but be better once he improves.
If it does get the hang of it there’s a few options to progress. Most of the youth are into trials and street riding but hockey and mountain unicycling are popular too. It can be a good sport, certainly no more pointless than football or mountain biking.