He didn't take the gun to find Ian and use it on him or threaten him, it's for safety. And he was just a regular person before being involved with an ex high ranking gang member. It'd be surprising if the emotions weren't high with how non-committal Ian acts. Not that it's his fault, but the regular citizen that came to love him is being lead on a leash while people from the underground are coming out around him. He's confused and impatient, he wants Ian to not be in that dangerous life but doesn't fully understand that just choosing him won't take him out of that world, and that Ian doesn't want to bring him down to that world just to be together.
Also, genuine question, what makes it seem like he's obsessed with him? Is liking someone and being concerned for them obsession? He comes to like this guy a lot and finds out he's sleeping with other people, but doesn't want to give up on something special (in his eyes). I have not seen anything Jo has done that is anywhere near the manipulation and abuse TJ has done to him, so I wouldn't say Jo is "similar or worse". He literally had Ian's legs broken to keep him from leaving him. I'd say TJ has a dependent obsession with Ian rather than Jo.
Finally, someone that reads with there eyes open
Why is Jo the best option to Ian I don’t understand? Jo is creeping me out. Ian throw a tantrum and left TJ cuz he didn’t want all that “bad things” I suppose. But Jo is not better. He creeps me out I feel he is way too obsessed with Ian, I mean taking a gun to go and find him? Supposed to Ian run away from that kind of man (TJ) only to find someone similar or worse? Jo doesn’t show us exactly what kind of character he is and it’s scary to me. And tbh Ian can’t escape that life he used to have with gangs and all that. He just have to accept his fate.