I mean I would prefer the junk food as well if I had the choice.
Well yeah

but that's not the point. We know the facts, they don't. We can see the long term implications, they can't.
I think kids-specially boys-don't have such a relation to their weight like adults.
Agreed. That's one of the reasons why I've always promoted the health side with my lads. I'm not a great one for official talks around the table about this sort of thing. I prefer to add snippets as we go along.
for example, when they were younger I might ask them why I've put broccoli on their plate. It's just a passing comment, not to get them to eat it, but just a mention that they have broccoli to help prevent them getting cancer later on....etc etc. We even have a joke about eating certain things to help them poo easily
I don't remember ever mentioning that they should have this or that to keep their weight in check. I think I just felt that the weight thing would deal with itself if I could make sure that they had a healthy relationship with food in the first place. After all, though you can get overweight on healthy food, you can also eat badly and stay slim. It was more important to me that they enjoyed good food, had energy and were eating foods that prevented future problems.
It's an ongoing education for them. When we shopped, they chose the fruit and veg for the week. They helped prepare it. I talked it through with them. How good their choices were because it would make them feel good in the long term.
That doesn't mean that we always ate healthy food. They get sweets sometimes and puddings, but they know that it's not something that they would want to do regularly because it serves little purpose. Not a treat as such, but just something that we have every now and again. No big deal. I've even made a laugh of it. "Right boys, and what is this doing for you.....sweet fanny adams....

"
They are both very healthy. rarely get colds etc. I do think we are lucky though, but always take the opportunity to tell them that it's because they eat well
When DS1 entered his teens, there was a spell when all his friends started to the McDonalds visits way too often. I do remember turning a blind eye sometimes, but did tell them about how good the McDonalds advertising is to attract kids. My main answer to "why can't I?" is nearly always "because I love you". Can't argue with that
Do you think something like 3 lunch packages and 2 times free choice at the cafeteria would work?
It's a good start. Don't bow down though. Stick to it. Let him know it's because you love him and you can't bear to see something so precious get harmed.
BTW, my kids were never fond of sandwiches, so rarely had them. I did try to put lots of little things in their lunch boxes. It takes a bit of organising at the beginning, but you soon get into a routine.