Ey bro,
I agree with mikzimous and chernandez1. A good talk will go a very long way. Let me just add something. Try mo i-apply to: Kiss-Kick-Kiss.
It's a format which works for me in encouraging younger musicians to play better. Basically, encourage mo siya with a positive about his playing. Make sure to cite particular examples to show that you actually are listening. Then show him what can be improved. Then encourage him again either by reinforcing what you said first or by giving him something to look forward to.
I've been using this to "musically disciple" some of our younger musicians and singers to great results.
Example:
(Kiss part) Bro, nice playing today! Ganda nung solo mo dun sa "I Belong to You" tapos swak na swak yung sa "So Good".
(Kick part) Let me just point some stuff out na pwede pa nating i-improve. First off, dun sa "10,000 Reasons" medyo off mag shred dun sa instrumental part natin. Mas ok sana kung kaya nating i-reinforce yung melody. Tapos dun sa verse ng "I Belong to You" pwede kang hindi muna tumugtog. Mas masarap pakinggan ang dynamics ng song pag may nawawala.
(Kiss part again) Again bro, I love the way you play! In fact, may mga nagagawa ka na hindi ko kaya. The way you worship is encouraging and I can see you as someone other people can emulate.
Had this conversation with one of our younger guitar players before. Di ko lang maalala yung exact songs at yung mga details but you get the idea. He's honestly a better guitar player than me, medyo kailangan lang i-tame yung crazy guitarist side of him and let the worshipper come through. Now, he's one of our worship leaders in the main church and a worship team head/trainer in one of our daughter churches. He's also become one heck of a guitar player (though I still talk to him about the amount of gain he uses).
Remember, discipline out of love. Walang natalo sa nagmahal.
Hope this helps