He’s 16, inconsistancy is normal. Watch some of van der Poel’s early CX races, and you can see similar things.
I do get it is frustrating for him (and you) though, so using that as motivation to change something. This might be a good time for him to learn about nutrition, and to work out a good warm up and pre-race food protocol.
If he’s adamant its sleep, maybe see if you can track his sleep with a watch or even just a phone app? Even if its not 100% accurate, it’ll give you some more information, and might convince him that its not sleep and he has to look for something else.