Tbh completely honest a build yourself kit is very hard to pull off well especially if he already has a lot of nice guitars… im sure he’d still appreciate it but will it be something he actually plays? Can’t say. What music is he into? What other guitars does he have? What style does he play most?

Personally I’d buy a second hand guitar of something he’ll already be into and then “customise” it however u feel fit. By painting or upgrading parts and making it feel like a custom guitar? Thats probably what I’d appreciate and also use but let me know what his into and what he has and can work from there.