Hi Stephan
First of all you did do this right. It sounds to me, in your short question, that there is more than one problem with your relationship. First of all, the more you ask or beg him not to drink or get high the more likely he is to do it. It's like a parent telling the child not to do something and the child will do it anyways. You knew this when you got into the relationship and now you want him to change. It just doesn't work that way. He will rebel and do it more. Second, it sounds like you feel a little insecure with yourself or he is insecure with himself and likes the fact that you do get jealous because that is showing him that you care and that maybe what he is looking for. Trust is #1 in any relationship and without it there isn't a relationship that will last.
Personally what I would do is just relax and trust him unless that trust gets broken. When he comes homes after a night of partying and calls and tells you about all this flirtatious stuff just tell him that it sounds like he had a nice night. Don't get into a back and forth discussion with him. Don't question him and start arguing. Even though it may take a lot on your part not to feel jealous he will start to get the hint that is doesn't bother you but make sure you do reassure him that you love and care for him. He might just need your reassuring words and telling you stuff like this is his only way he knows how to get this. You reacting is showing him that you care. So, trust him 100% unless he has absolutely done something to break that trust. Once the trust is broken then you have bigger problems because sometimes that trust can not be brought back.
Communication is also a key in the relationship. Talk with him about how you feel and vise versa and then let it end there. No need to get into talking about getting drunk, high, flirting on a daily basis. Let him know how you feel and let it stop there. Being worried in a relationship all the time really sucks for both you and him.
Good luck to you and I hope it all works out. SusieQ