Not a doctor but seems plausible that in collisions where neck and spine may be affected they leave helmet on in case a tug or pull to get it off can risk further damage. If just a knee blow to head they might be confident brain injury only and can start attention easier and the helmet taken off isnt a risk. **Remember I said plausible just from observation** I am no doctor clearly.
Either way I agree wish the best for the young man and his family.