how to handle this situation?

[PoliteViper]

Hello. My son is now 20 years old and is still living with me, as he is studying. i am a single father who has to balance work with being there for him as well. i try to have a very generous approach to raise him. that means giving him a lot of freedom and not much boundaries, especially as he is now 20 years old and therefore definitely not a small child anymore. having said that, he can be very irresponsible, especially when it comes to health. This weekend he had an injury where i needed to take him to the hospital. turns out that he has torn the ligaments in his ankle. the doctor said that he would need to wear a cast for 2 weeks. but already yesterday i saw him without the cast. he just said that it is annoying him too much and it would not help anyway (!). i tried talking to him and convince him that it is not a good idea to take it off and that he has to listen to the doctors, as it may postpone recovery and may cause long term damage. but he just would not listen and do leave the cast off anyways... i am scared that he might cause himself some long term damage, so i am struggling to just keep watching. one part of me wants him to be independent with his decisions and experience adult life but the other part of me wants to step in and protect him from reckless descisions like these.

i have to say, that he has lost his mother/my wife some years ago. in the begining he could deal with it quite well giving the circumstances. he also would never do things like going against doctors advice like he is doing now.. so i am a bit unsure, if he acts like this because of the loss of his mother or if it is an age thing or if it is something else.. having that said, i really struggle to know how to approach this situation. should i just keep him letting do his own decission as he is an adult, or should i step in (firming up on him), to protect him from bad decisions?

i really tried to talk with him about it, but he does not care at all, i am not getting through to him...

how would you handle this? i am really greatful for your thoughts!

UPDATE:

so as mentioned in a comment before, i now just set up an appointment at the doctor for him (which he was supposed to be doing). i confronted him with it, what date it is going to be, that i am going to drive him and be there with him, and that i would not let him get around this one. i explained him, that i only do it for his own good, and that i am just massively concerned about him.
to my surprise he did not resist at all this time. he just got quite emotional and i could feel that there is something more to that. so i guess it will take some more time to really get to the root of all this. he also seemed kind of relieved(?). i am just happy that he accepts the doctors appointment now and we got a step forward... phew...

This is it. At 20, you can offer advice and guidance, but you can't make his decisions or force him to comply with medical treatment (and nor should you try to).

Everyone makes mistakes during their early adulthood, and it's part of growing up. Valuable lessons are learned, and the process contributes to a fully-formed, sensible (hopefully!) adult emerging at the other side.