Given the cost of uni, it would be mad to encourage him to go solely in the hope he will make friends. If there was any actual link with a career he was interested in that would be different.
However the combo of no full time job yet and living rurally and no uni means it will be unlikely he will have much opportunity making friends unless at least one of those circumstances change.
How rural is rural? Is there a city within reasonable distance? Could he try a houseshare, both to meet people and to be somewhere where there are more activities. Depending on what he's into joining clubs is the best way to meet people his age - doesn't matter whether that's d&d/larping or 5 a side football - there will be options in most cities.
What is the part time job, could he swap to one with more young people e.g. Mcdonalds/local cinema?
How about volunteering somewhere?
Would he be interested in going travelling? Lots of places organise group holidays if its too intimidating to go all by himself, even if he doesn't make friends for life it will at least give him confidence that he can make friends. Again lots of variety - doesn't have to be a piss up 18-30 if he's not into that but could be climbing/adventure sports, or some sort of voluntourism .
I'd also have a discussion with him about responsible spending -unless you are absolutely loaded spending £120 on teacher presents is insane. If he's done a levels he would have only done 3 subjects- there's no way the friendship groups would have put in 40 quid each per teacher, and no teacher would want or expect a teenager to give them anything costing that much - if he really wanted to get them something a bottle of wine or box of chocolates would have been more than enough.