Reasons a 13-year-old still can't use utensils properly?

I dont know.......r u sure he doesn't have autisim........? I dunno......but 

yeah

I guess

but ont blame him unless uve got a lot of reason to

I think the Autism is HUGELY responsible for why your brother still can't feed himself correctly. He needs to be retested, because it's possible that his symptoms may have increased since he was first diagnosed or identified as a person with Autism. If so, he needs to be in therapy (such as CBT) which will help him learn in ways that he can manage and which are adapted to his particular level of functioning.  NO, this is NOT HIS FAULT, nor is it his mother's fault. People with Autism disorders are NOT RESPONSIBLE for their disabilities. They DON'T ASK to be born with them. I'm mildly Autistic myself, and I can imagine what your brother must be going through, especially at his age. Think about what life was like for YOU when you were his age, and then put yourself in HIS SHOES and ask yourself if YOU'D want to be treated the way he has been treated by almost everyone around him.

Autism has NOTHING TO DO with a lack of discipline. I'm only mildly affected myself, but I had a step parent who thought the same thing that your family does about your brother about me, and used that as an excuse to nearly kill me more than once while I was growing up. And he justified his actions by saying that he was trying to "train" me. I almost died- and worse, I was left with psychological wounds that will NEVER HEAL completely,no matter how long I live or where I travel in life. Your family's mistreatment of your brother is undoubtedly causing him the same kind of pain, but he evidently isn't verbal enough to be able to say anything about it. I feel for him, I really do.

You need to do everything you can to PROTECT your brother- because he needs your help, and he needs an advocate. People with Autism often have issues with coordination and fine motor skills, and it's not uncommon for them to have issues with sensory integration as well. That's because their brains are wired differently than those of neurotypical people. If per chance your brother has an intellectual disability in addition to Autism (and yes, ID and Autism can and do occur together) then that might also help explain his inability to follow directions and to grasp simple concepts like how to hold a fork or a spoon.

This is NOT a disciplinary issue. It's a MEDICAL issue. Your brother has a disability.

If he has autism that will have a lot to do with it (although I have autism and can use a knife and fork just fine and so could my ex who is also autistic but then we both have mild autism). Does your stepbrother have quite severe autism? 

Is he filipino?  In my house which is a filipino one a lot of the food we eat is done with our hands, i cook a lot of filipino food meant to be eaten with your hands...if you sit there with a knife and fork you’ll look like a fool.  Although if y’all are not filipino...using a knife and fork seems pretty standard practice.  Just continue with repetition.  When i was young all the meat i ever had was very tender i could cut it with the side of a fork so i never used a knife till i was like 12.  My parents trained me to do it though.  Every night for 2 weeks we have food that i need a knife for.  It’s the only way to get it right is through repitition

It's called cutlery and Americans don't know how to use them anyway. Hes hardly going to stand out.

he might have some kind of disability that he needs to get checked out

Poor boy, I've heard of people not learning how to use cutlery properly until leaving home because of overbearing parents doing all the cutting up for them, but this is different.  It's possible that by now your stepbrother has so many bad associations with using cutlery that he has some sort of mind block going on if his motor skills are otherwise okay.  If you want to help him do you think that you could make fun games involving picking up and carrying things with a spoon and then progressing to a fork?  It might help if there's no pressure of a meal and expectations to sit nicely iyswim.  It's a shame that your parents won't meet him halfway and serve more dishes where everything is already in bite sized pieces.  That might be less daunting.  Maybe you could do that if you take a turn cooking dinner?