I've helped a lot of people dealing with family members with addictions.  You are in a near impossible spot as you love them and you want to help them but really don't know how.  Frequently that help hinders them and their ability to climb out of the pit they are in.  Might I suggest a few things that might help you:

1. Remember that addiction is a disease, so treat it like cancer - love the person, hate the disease - but realize that while poor choices may have lead them to where they are, they are likely no longer in control of those choices.  If you can see things this way with them your approach will be better for both them and you.

2. You are his father, but you are not responsible for curing him.  People supporting addicts frequently get upset when I suggest reading this book, but are glad for the suggestion after.  Read the book, "Codependent No More".  Most everyone has codependency issues, or you may have a spouse or someone else who does.  But this is a great book that help you understand what is and isn't your responsibility.  It boils down to basically keeping your own life in order and letting them get theirs in order.

3. Addicts lie cheat and steal - he may deny harder drugs or alcoholism, he may tell you everything you want to hear.  One of the hardest things you may do is to not help them in any way, other than things that are going to be productive, which at this point is basically rehab.  Don't give them grocery money, don't buy them a burger, etc.  

4. And the hardest thing of all - in context of all of that - increase your love for them and realize you not helping them in the ways they want is maybe the highest form of love, loving them enough to hurt them.  I've seen God work mightly miracles in individuals I thought were completely lost - but it began on the basis of love (and typically hitting rock bottom).  

5. Consider joining a support group.  If you are LDS there are support groups for family members of addicts through the Addiction Recovery Program.  Talk to your bishop and he can give you information.  They have groups that meet in person (pre-covid) and call in numbers you can call in anonymously. 

You aren't alone.