That's awesome!!! It sounds like his goal of low 17's by the end of the season is attainable.
Just some thoughts:
Look at your schedule starting at the district meet and work back to today. Figure that you are probably going to have easier workouts the day before the meets. Hopefully, you don't have more than one per week from here on in. Fewer would be even better. Pick which days you could actually get some of the more challenging workouts in. It's too early to taper yet but you should be starting the process of working on end of season goal pace in some of your tougher workouts to allow time for adaptation. Two weeks from districts you should probably start backing off of the volume a little but crank up the pace with more recovery in your workouts. You have to do some training sessions faster than goal 5k pace. Start training him like a miler in some of your workouts.
I would suggest you plan long intervals, easy runs, repition/speed work, etc... in such a way to allow for recoveries that will add up to continued improvements in race day effort. Course rigor will determine whether a fast time can be achieved or not so focus on racing athletes rather than running goal times. Most kids that are competitive are in good shape by this time of the season.
By your conference meet you are going to want to have already looked at the competition and know where you want to place based on who has been doing what recently. Top 10? Top 5? Maybe even running with the leaders and letting the chips fall where they may? I don't know your athlete or your compeititon but setting goals based on place are a good way to focus on effort vs. worrying about running a time. Those tend to take care of themselves if you have been progressing the workouts. You already know what is possible by where he is with his training. If you can train your athletes by the watch and teach them to compete on race day, you have your best chance at a great performances during the championship season.
This is the most exciting time for coaches. Your athlete was willing to do the necessary work prior to the season starting. You have had success and showed great improvement in your first few races. Now is the time for meticulous planning but also using your ears and eyes in the art of coaching. Know when is enough, err on the side of caution to prevent injury or fatigue and overtraining your athletes. With experience, you will know how far to push your kids and allow the proper recoveries that will give them a chance to compete well. Get them involved in the decision making process and ask questions that will allow them to process their feedback so they have some ownership too. Don't look too far down the road yet, there's still a lot of work to be done. Yes, a plateau could occur but that's not always a bad thing if you are still in a heavy training load. With rest at the right time, the adaptation could be tremendous.
Promote good healthy habits. Sleep, Eat, Wash Hands Frequently, Don't Think About Running Too Much Outside of Practice etc...
Focusing on racing also will take the edge off of the changes in the weather that are going to start to occur. Kids that don't do any summer training have the hardest time with this. They are just starting to get into some kind of shape and when October comes, they get squirrely when the air starts to get crisp. However, you do get to regain your sanity after the conference meet when their season is over if you have a focused varsity squad.
Anyway, best of luck and no matter what happens, this could turn out to be a great learning experience that will really benefit your athlete for his senior season. A take-off point if you will.