It's a needle you have to thread all the time as a coach. Not sure how it would be coaching boys because I only coach girls, but I try to acknowledge the progress, congratulate the result, but then challenge them to be more aggressive, run with more confidence, etc. in future races. Paint the picture for them - "Man, with a taper, fast course, cooler weather, strong competition to pull you along, I KNOW you can go even faster!"
The good thing about XC is the regular season meets are opportunities to try new race tactics, to find that line of going out too fast and dying. We take splits at mile 1 & 2 and see how much they slowed or sped up, what was their position in the race (they get tiny skull beads if they improve position) so you can see how they actually did versus their competition. And when you have familiarity with competitors or teammates, you can challenge your athlete (a few days after their race) to stick with or pass athletes they KNOW they can beat or keep up with. I tell my girls all the time - "you did the same workouts as Girl X, and she just popped off a great time...so you can do the same thing."
Track is also quite different from XC - so you need to understand do you have a grinder, a kicker, a "start fast and hold on for dear life" kind of runner or what. I encourage my girls to tweak race strategy each time to see what formula works best for them.
I have 2 of my slower girls who have really started to buy in and they both set big PRs recently...but they did so with a very conservative start and a strong close. This next race, I'm challenging them to start a little (10-15 seconds) faster and see if they can replicate their miles 2 & 3 from prior race. A LOT of it is just confidence - if they've seen they can finish strong, they can run a little more relaxed - yet faster - early on because they KNOW they have that ability to finish well. They feel less pressure to bank time because they have doubts about closing well. Doesn't mean they'll do it, but if they THINK they can they are that much closer to actually doing it.