It's certainly possible that he's got a slight shoulder strain or something - 3 very tight, important 3-set matches plus the doubles matches must take some toll. In fact, it's amazing how many really tight three set matches there's been in the juniors (well, maybe not amazing, but note-worthy, especially given the difference in rankings/expectations/supposed 'worth' of the players.
In other words, the scoreline is actually deceptively in Kyle's favour. If it's simply a case of "Zverev played better on the day", I'd be amazed.....those stats (given Kyle's pedigree and the fact he's 2 years older) suggest a problem somewhere. Hope we find out.