You are his uncle. Are you the "cool uncle" in his eyes?
KIds tend to hang on every word of their "Cool Uncle", as opposed to listening to the same old crap from their parents and siblings.
Use that to your advantage. Take him to do cool stuff, then sidle into some of the things which he would be benefitted by if he would take the time to listen and try. You may be the only one who has his ear.

X2

Dad would say four years in the infantry in ww2 and you’ll eat no questions asked

Id really like to get him to help with cooking. I've tried before but and his momma is so dang over protective. Worried about him cutting himself or if I show him out to cook an egg or something (This word is unacceptable on Trapperman) som how manage to burn the house down. Can't do much around her honestly. Gf was the same till I sat he down and had a really deep talk with how life is as a guy and how he's gonna have a really hard awakening one day if they dont stop babying him. Especially if he goes to public school. But now that j get him away from the house I can actually do something with him. And helping him with boy scouts is a perfect excuse now. Will say though pretty worried he's gonna puke or something when he sees what food looks like before it cooked raw chicken and the like because when she does cook they mom doesn't like them in the kitchen.....mean while my parents had me in there working like a dag slave

I guess I am . His other uncle is kinda (This word is unacceptable on Trapperman) and looks at kids like a lot of people look at annoying pets. That being said when I'm around apparently he acts a lot different and listens better according to my gf and sister in laws.might just take a few more trips till he starts lookin at me like the cool uncle lol

So, if I'm understanding this right, this is a kid of your girlfriends relatives and not your kin?

As I said earlier, I still don't think it's your place to try to fix his eating habits. But if you insist...

I'm a picky eater, always have been and will be. Now, I may not literally starve myself to death but I bet ya I could fast for better than a week before eating most veggies. Because what non-picky eaters don't understand, is it's better to be hungry than eat something that will make you throw up or gag.

But, that's probably not the case with the kid in question, he's probably just spoiled. Nothing wrong with forcing him to try one bite. If the threat of going to bed hungry doesn't do it, the threat of taking away his phone/tv/game/boy scouts/whatever probably will. If he tries a bite of something and pukes or gags, well, duh, don't make him eat that again. If he tries a bite of everything and says he doesn't like any of it, well fine.... he did what you asked. Just thank him for trying and let him be hungry. There are thousands of other foods to let him try the next day.

Does he have allerdies?? Had a grand daughter that was a picky eater found out that certain foods made he sick, stomach aches, nausea, headaches . Found out allergic to gluten and dairy products.
Allergies have since " disappeared and now is an eating machine

What school does he go to now if not public school?

Spending time with him will benefit you as much as it will him. I have done a lot of that in my time. Little league baseball, football, basketball, Cub Scouts, Boy Scouts, and 4-H Rifle Team.
I always found that I was drawn to kids who didn't excel right out of the gate. I figured that the kids who were natural talents didn't need me all that much.
I never talked down to kids, or show anger or raise my voice. I spoke to them on their level. I pulled them along rather than push them. That tends to lead to a buy-in on their part. After a while they would hang on my every word, just like how a kid does with his favorite uncle. I gave everyone a nickname. Put some thought into each one of them. They ate it up.
There is nothing that I find more rewarding than to see a kid finally get it. I'll never forget the smiles on their faces and neither will they.
Many of the kids are from broken homes or their dads are away from home in the service, and their moms want them to have a male role model in the kids life. These kids, though problematic at times, are starving for someone to work with them and be praised when they do something praiseworthy. I miss those days.