A basic DI box into the PA will get you going. The ADI 21 is a good way to try some tone shaping, but worth buying just as a DI box even if you don't turn it on. If you have reasonable stage monitors, that's all you need.
Depending on how loud you are, and if you don't have any on-stage monitoring from the PA, a small acoustic guitar combo might be worth considering. I have a Roland AC60 which is now discontinued but can be found second-hand. There's also a 30W version, the AC30. It has an anti-feedback filter and other effects such as reverb and chorus, and best of all it can be stuck on a speaker pole so it is at ear height and he'll be able to hear himself.