Please don't do this.
The pairing of khakis with a light blue Brooks Brothers button down shirt is the ubiquitous uniform of the unimaginative office drone, seen in airports and cube farms the world over. Even if you're starting out on your sartorial journey and taking small steps, a little bit of flair or imagination will set you apart from the crowd in a good way.
Agreed that it is ubiquitous, but you have to start somewhere. Telling someone who is just getting started to go with some flair is usually going to end in tears. Get the basics down, then move onto something more advanced. He asked what shirt to wear with the trousers he has and a sweater. I don't think there is a better answer to this than a blue, white, or blue/white striped OCBD, especially for someone just getting started.
Granted that some better trousers would do wonders though. Not a bit of argument there Some mid-grey flannel trousers and nice shoes, and assuming that everything fits and the sweater is tasteful, would make this a legitimately nice outfit.
And as always, fit is paramount. If those OCBDs don't fit him, they will look terrible.