For a beginner, I'd get a decent, pre-built guitar amp that "just works" with decent sound and only basic controls (as opposed to a digital amp with millions of mediocre effects). You can always add some DIY effect pedals, or get a whole new amp - but if your son wants to practice and needs to fiddle with the electronics because something is not right again with the DIY amp, this might distract him too much from actually practicing his instrument.

Maybe take him to a store so he can find out which guitar/amp combo he likes most.