Crossfit is not training, it is exercise. It is also self-imposed torture, the kind that leaves you maimed and with PTSD.
I jest. If he's after the "pump" or the "burn", he's gonna keep doing whatever he's doing and, from my experience,
you can't convince a crossfitter to stop doing Crossfit.
However, if he is looking to get stronger, you can try to explain to him the difference of training and exercise by using this article:
The Biggest Training Fallacy of All
If you can't, keep at it. Eventually he'll see that the weight on the bar has not moved in months and be more malleable then.
Best of luck.