I've never heard of that sort of lingering soreness from stretching; that suggests to me either that he's never done any sort of serious stretching at all before, or that he's being overstretched a bit in practice. Of course, its also possible (and in fact probably more likely) that the soreness is more from strength work or other parts of the workout. Perhaps he did more running, tumbling, plyometrics, pit laps, or something like that, and just thinks it's from the stretching.
At any rate, I would recommend gentle stretching at home. Daily, if you can talk him into it (though don't push it if it gets to be too much of a chore). Also a nice warm bath/shower after practice will probably help. If a warm bath/shower followed immediately by stretching (while he's still warm from the bath/shower) becomes part of his daily routine, that will probably help a LOT, regardless of whether the soreness is coming from stretching or conditioning.