If he likes to initiate conversations with you, which, after you reply, he is going to pretend to forget about it is a red flag.
Negging means giving someone a backhanded compliment. These are insults that are delivered like a compliment.
Trust is a big thing in any relationship. One sign of missing or broken trust would be his suspicious behavior whenever he is on his phone.
If he is a master at flirting and does it in front of you, that is also another sign that he is a player.
If he hides you all the time in your place or anywhere else, it's probably because he has no intention of ever letting anyone know about you.