Your son sounds a fair bit like my daughter, albeit younger, and whether you can learn from our experience I don't know, but here goes. DD had piano lessons for about a year when she was 9: sat Grade 1, got a distinction and then announced she hated lessons and was giving up. She did not sit at the piano for about another 4 years, but now, like your son, spends hours at it, jamming and improvising tunes she has heard (also Adele, but also lots of other stuff). She gets enormous pleasure from this, and has also taught herself to play guitar and ukulele. As she is doing so many exams at the moment, it had been a great escape for her, and I have been really pleased she has this outlet at her disposal. I think if we had done any pushing, she would not get the obvious pleasure she does from it...I don't really care whether she is technically brilliant, it sounds damn good to me and she enjoys it.j
Regarding violin, she took that up at about your son's age and had a teacher who was very keen to put her through the grades..at the rate of one or 2 a year. With each exam she got progressively more nervous, shaking and even dropping the bow once, and more of less passing out with nerves another time. Though she passed, the marks went down each time, although her teacher was of the opinion she was doing amazingly well and should have been flying through. We did try to stop the grade machine quite early on but it seemed to be a treadmill it was hard to get off, but after grade 5 we decided to call it a day grade wise and she has continued to play with markedly more enjoyment. I don't know what grade she would be now as we asked the teacher to lay off all that kind of stuff and concentrate on playing for pleasure.
In short, you don't have to do grades, nor ask for them to be 'pushed on' if your aim is that they will enjoy playing, be prepared to experiment and just love music. It will happen by itself and you could kill the spark if you try too hard to fan it.
Like you I am no musician, but for our family, this approach has been the right one.