I've grown at least ~2 inches from when I was a junior from 5' 11" to almost 6' 1" so he will be fine. Given I'm 22 now but you get the idea lol.
As long as he is aggressive it won't matter if he's 5'4". You get into the WR off the line and disrupt his route and you win the battle most the time as a CB unless you are dealing with a big, physical WR.
I think you're looking at the wrong Rivals......