7) His friends do not know you and your friends see him with suspicion
Now that's a real check point. He never introduces you to his friends. He has never sent you a friend request on social media. He has never even told his friends that it is you with whom he is in relationship with, these days. You might be around; still he behaves like normal friends and no special treatment is given to you. In simple words, he behaves like any other person in social circles with you.
On the other hand, your friends do not think the way you think about him. They are either too quiet on his mention or telling you to be careful but for sure not with you. (A good enough sign – something is wrong!)