Nobody wants him participating in full practice until next season ( i surely dont). But, light skating on his own, to activate his quads, cycling for his hams, etc, to combat muscle atrophy will only help. As long as he's wearing his brace to avoid any twists, etc.
He's over 4 months post-op, so he's almost there, if not already with fullbone growth around the pins holding his new ligaments in place. That's the biggest hurdle really. Odds of him re-tearing after ligamentization vs tearing it out of the bone before bone growth and fusion of pins /bones is much lower.