Hi there. I'm a mom and have two boys. One eats healthy and one tries not to. My theory is that I'm the shopper. If I don't want them to have soda, I don't buy it. That really eliminates some of the bad food choices. and I think that they begin to develop habits. Now, I hated vegetables as a kid but as I got older, I developed a taste for them. So, our pallet does change as we mature. I loved tv dinner spaghetti. Now? Yuck. But I had that for dinner regularly because I'd eat that and not complain. Guess my mom took the easy route at times. And I take the easy route too sometimes, to be honest. We order pizza a lot.
But I also know that I have to instill in my kids the idea of a health lifestyle. I need to model eating properly and exercising. I need to make the good food choices readily available and limit the bad ones. I need to also try to make some choices that they prefer more healthy. A hamburger isn't bad if you make it at home with lean ground beef, whole wheat bun, lettuce, tomato, etc. One that is from a fast food place loaded with sodium and fat? Not so good. So, you can play around with what you serve to make it healthy.
And I found that as my kids got older (now 15 and 14), they developed goals eating wise. Both play sports or run competitively. So, they want to be healthy and have proper 'fuel'. Honestly, my picky child got advice from his coach that was impactful that I could never give him. Sigh. He eats a spinach salad now often and various other things he wouldn't touch with a 10 foot pole before the coach talk. He also has GI issues that are long standing and has found that his hydration and eating impact that. That is motivating to him as well. My other son thinks about things like what will give him the most energy without being heavy before a game so he started thinking about his food in those terms. It helps when they get involved themselves.
I would say that habits of eating terribly without being mindful as a child can lead to a greater chance of being obese as an adult. And kids have to go through the sugar withdrawals just like an adult does. So, it's not easy to get on track.
Make a menu weekly, let him help you prepare food. You take control back over and he should start to benefit from that.
Oh, and my older son has sensory issues and motor planning difficulties which have always impacted his eating. Chewing meat was very difficult and he would choke. Textures and tastes created issues. We worked on it. Dips help as all kids like to dip. Like ranch dressing with some vegetables. Dip into dressing, hey, he's eating carrots. It's a start. There is also a book called "food chaining" with instructions of how to increase what a kid will eat. I found that really helpful.
If he truly eats like that all the time -- and if you're not living at his house it may be you've only seen what he does sometimes, not all the time -- it would be odd he's skinny unless he's super active all the time. But reality is, different people have different consequences of their behavior, and some of the bad things that happen to people because of their behavior might not show up until they get past 40 years old and it starts to hit home. At the least, he should be having lots of teeth problems, but again, he might have really strong teeth. So there's no definite way to know who can do pretty much anything and feel great and live to 100 and who can't. He's certainly playing the dangerous side of the odds, though. Does he have parents? Because if he's eating this way they're either incredibly disengaged parents or they eat this way too and so you can see what their health is like. Are they obese? Are they sick a lot? Have blood sugar problems? Bad teeth? That might tell you where he's headed, but again, you don't know if this will continue or if he'll change his habits when he gets older. Wouldn't recommend how he lives, but also can't predict how it will all turn out, either. If you want to play the odds, you wouldn't do this, but again, he's the only him.