If you are just talking about speakers, then it's a pretty easy solution. Some speakers have 2 inputs and will output sound from whichever input they are receiving from. If you're considering headset audio as well, then that becomes more complicated, or more expensive.
Controller pad and HDMI aside, I think the only other audio output from PS4 is optical, so you'll likely need a DAC (digital to analogue converter) to which the speakers will connect. Some monitors have an audio output jack to which external speakers or headphones can be connected, which you could use, but the quality from those outputs is generally not very good though to be honest. Using a DAC connected to the optical out will provide much better quality.
Some speakers do have a DAC built in, but the choice is small and possibly expensive; depending on what you're looking at spending. A cheap DAC means you can connect any stereo speakers or 2.1 speakers.
It really depends what you're looking to spend and what kind of space you have for speakers. I suppose his age comes into it as well. If he's very young, he may not care so much about sound quality as someone older might do, just as long as it's loud and decent enough.
Creative T20 or T40 is a good place to start. Small footprint, 2 inputs (one rear, one front) and pretty decent quality for their size. If he's very young then they'd be ideal really.
If he's older and can appreciate better sound quality, then if space is not a problem, something like Edifier R1600T III might be a good choice. Like the Creative's, they have 2 inputs; both on the rear. Although they do sound better than the Creative T20/40, their improvement lends itself more to music use than gaming I'd say.
2.1 speakers are also a possibility, but very few have anymore than one input, so you'd need to hook up a switch box or splitter cables.
