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I'm also an average sized woman with a tall, sporty, ever hungry fella who can eat twice my meals and not put on a pound. And I started off by doing exactly what you are suggesting - which is to add the fattier and higher calorie foods to his similar sized portion.

But the thing that worries me about filling up someone who can burn calories quickly is that the easiest ways to fill them up (sugar and fats) are no better for them than for other people. Yes, a cream sauce, fatty meat, or a big cake will give them the extra calories, and yes, they can burn it off and not get fat, but they are not safer from diabetes or fatty arteries just because they are burning calories. Blood sugar levels are still an issue. Your fella is no doubt coming back craving fat and sugar because it's a shortcut to the calories his body desires, and because it is what he is used to eating when he is famished, but it doesn't mean they are any better for him than someone who only needs 1800 calories. A balanced diet is still balanced when it is bigger.

So I know it is not what you want to hear, and feel free to ignore it! But my experience is that it is better long term to get used to having proportionately sized portions of the same balanced diet. Protein and good fats with complex carbs will help you both fill up and stay full for longer, he just needs more of them. My trick for this is that we bought plates from the same dinner set in slightly different sizes so mine is about 2/3 the size of his. And so we both have piled up plates of the same foods in the same proportions but his is just scaled up, so he ends up with more.

Another thing I found out is that eating a small amount of (preferably good) fats with a carb slows down its breakdown and promotes a feeling of fullness - hence the italians eating bread dipped in olive oil as a starter. So something similar may take the edge of his hunger when he comes in from work and mean the main meal doesn't get hoovered quite so quickly.

Good luck! We have been together a long time through illnesses and babies and the diet thing is always a bit of an issue, but with a bit of perseverance you'll find something that works and keeps you both healthy and satisfied. x

The big picture is that your grandson is starting school and eating a packed lunch with a huge bunch of other people will be enough to take in by itself, without also taking on new foods straight away.
So I would personally initially stick to familiar foods which are relatively healthy and which you know your grandson will eat happily, so that he can focus on the rules and conventions of school lunchtime. You don't want him to associate meal times with stress. It's 5 meals from 21 in a week, so you can afford to break it in gently.

As he settles in and gets the hang of it over the next few weeks and months you can start introducing more interesting foods, just at maybe two meals a week each week. You can also then start to think about a more considered balance of proteins (tell him they are sporty muscle foods) carbs ('energy' foods) and fruit and veg ('no sniffles' foods).
There are some good ideas for starters at www.school-lunch-ideas.com and www.kitchentablemedicine.com/55-healthy-school-lunch-ideas/

But give yourself and your grandson some breathing space to take on board a huge event - starting school - without worrying about whether or not he will be the most gourmet kid at the table. If he's one of the most comfortable kids at the table, you can add the other stuff later and he'll take it on board. Or you can keep it simple permanently and do the interesting stuff at home in the evening.