Prison is no excuse to use drugs - from a Dad of a prisoner

The words, "say no to drugs" is basically all that is left! 
The country is screwed and drugs are part of society. The only way to curb it is to legalise it. Some may disagree with what I have got to say but I have 2 sons. 1 turned right and the other one turned left which led him to prison. Drugs played a part in it and from what I can gather he is still using drugs in prison. He has been inside numerous times and I don't see him. I have put myself out for him time and time again and he still takes drugs and goes to prison. Every time he goes to prison he does a withdrawel and yes every prison has made sure his withdrawel is a safe process and I cannot complain with the care he has received in HMP. The support has been there for him so the choice is entirely down to himself. He doesn't have to use drugs in prison, but he chooses too. I cannot do anything about that just like I can't do anything about him using drugs in the freeworld. 
Our family has paid for rehab for him twice and it didn't work. What more as a family can we do because I'll be damned if I keep putting myself out spending money to visit him in prison when he has no intention of sorting himself out.
Family support is apparantly the way forward but I've worn the shirt, rung it out and dried it time and time again. I cannot physically or mentally 'make' my son stop using drugs. Until he's ready, I will remain to cut ties.