I agree, ensemble playing is a huge boost to enjoyment. It's all that's kept our daughter playing at times. She was a "music scholar" with the LEA music service during her last year at primary (in practice, that meant one-to-one lessons rather than group - and at a substantial discount to boot
) because she was considered to have a lot of ability and potential, but it was a constant battle to get her to practice and when she took G2 (last summer) she only scraped a pass. However, she moved up to a more demanding ensemble at start of Y7 and has really enjoyed that - she got high 90s% in G3 in December. (Mind you, I instituted a "Chinese parent" regime of 30 mins practice every day as a condition of pocket money/treats/meals/sleeping indoors instead of in a tent in the garden after the G2 debacle, and that's played a part as well.)
My opinion, which I worked very hard (but unsuccessfully) to persuade our son of, is that in later life you're never going to think "I wish I hadn't learnt to play guitar/piano" - but you may well regret not being able to. But they're basically solo instruments; it would be well worth your son's while to take up an orchestral instrument or band instrument and enjoy the social/collaborative side of music as well.