Knee Pain, ACL

First things first he needs to develop good muscle strength and proprioception to ensure good control at the knee.

Has the player seen a physio at all? If not it may be worth getting him assessed to find out what his current functional state is (e.g does he have any specific muscle imbalances/weakeness that need corected) They can also assess how stable his knee is and decide whether a brace of some sort would be of benefit.

Its possibly worth checking with the specialist if they have any set rehab protocols already set for post-op ACLS. It may be useful to allow the player to use these as rehab guidelines pre-op as they should hopefully help him gain the appropriate strength and control required to keep his knee stabile.