Advice Needed: orphaned kitten refuses to eat

I rescued two kittens who look to be no older than 6 weeks old about two days ago. One of them has no problem eating, but the other refuses to eat. I force feed him kitten formula through a syringe, but I’m worried that he’ll be dependent and never eat on his own. Is it because he has separation anxiety? They had their mother, but unfortunately some piece of s*** coworker beat her and we haven’t seen her in over a day and a half, so we don’t even know if she’s alive and I was afraid he’d do something to the babies if I didn’t act fast. I found out he’s been secretly poisoning the water we left for the mom and other stray cats in the area. Will find a way to deal with him as I have no solid proof aside from another coworkers word, who is also a piece of s*** with animals.

I googled ways to stimulate his appetite, and I’ve tried feeding him fish-flavored kitten pate, warm milk, and both those mixed together. I haven’t tried chicken broth yet as I’m a broke college grad so I won’t be able to get some til next payday. Will he always be this way, or will it take time for him to gain an appetite? He plays, sleeps, and pees just fine. I haven’t seen him poop yet but he’s only been drinking formula so I’m not sure if that’s something to worry about. Any advice?