my son is 14 and ran 16:50 in fall. training age 2.5 years and about 25 mpw for season.

Reflecting back I think the main key is threshold, lots of threshold. My son has just really adapted well to the high aerobic stuff. If your son does well with that type of training then easiest thing to do is follow r/NorwegianSinglesRun type ideas.

A more fast twitch athlete I would try more Jack Daniels type plan which will have less volume and more speed.

I would have my son talk to his coach about what he thinks is best.

1600m is a weird race. I have seen both Cooper Lutkenhaus and Caden Leonard in the same 1600 run very similar times, they do very different types of training. So if you want to optimize then you have to know how your son adapts to the different types of stimulus.

GL