Press the Left Button on Tempo Trainer
Put a Tempo Trainer in your son's cap. Set it at, say, 1.2 sec/stroke. Have him swim a 25 or 50. Take the TT out and press the left button twice, increasing Tempo to 1.18. Swim another 25 or 50. Take TT out and reset Tempo to 1.16.
Count strokes. Watch for roughness. If there's a big jump in SPL (more than 1 stroke every .04 sec adjustment or so) or he gets noticeable rougher, have him stop there and spend time acclimating to that Tempo.
This is far more scientific than sitting up in the water and spinning your arms.
SL x SR = V.
Spinning is all SR, no SL.
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