1. He says that he's coming out of a long relationship so he has to go slow and take his time.
Or, in other words, he is not really going out with you! I'm sure there are exceptions to this but I have not heard of it yet.
If a man is comfortable with the idea of having an appointment with you even if he is coming out of a long relationship, he should not need to say "let's go slow". He would, simply, respect the fact that you are going out together and that you are not just having fun when he has a hole in his schedule.
2. You see him less than once a week during the first month of your relationship
I'm sorry but it's just not enough girl. Even if you try to go slowly in the beginning, if you see him less than once a week and you would like to spend more time with him, he is telling you that for him, you are not a priority. Whether he is not feeling so much affection to you or because his schedule is too busy is not the issue. The question is, you should go away or RUN!
3. He tells you that he has already had problems with drugs
He says that everything is under control today, so do not worry if he takes just a little, just now and then.
I will not even explain, the answer is simply NO. The situation is sad and I encourage you to encourage him to find help. But you are not the help in question.
4. He refuses to leave the smallest thing that belongs to you in his apartment.
Suppose you have managed to put the foot at home but this won't help you in any way, this is a very bad sign. You're not moving in just because you left a liter of soy milk in his fridge for the mornings when you wake up at his house and steal a coffee! You gotta do more.
5. When he says, he's all mixed right now and that you probably should not stay with him.
I think this one clearly says what he's thinking of, he clearly doesn't want you in his life, so leave him before he does the same.
6. He is everywhere on social media and yet, incredible fact, there are never any hints or photos about you.
If you a part of his life, you should be a little bit in his virtual life too, that's how it works in 2019. We can pass in the exceptional case of someone who does not post anything absolutely anywhere and never.
7. He never has money so you pay for anything.
I understand that we are in 2019 and that it is reasonable for the woman to pay for her food or even more if she feels like. But there is a difference between paying your share and paying everything. In general, when he "forgets" his wallet for the third time, and you are supposed to understand the "poor" guy.