Banana is the answer to all your problems. Because it adds a lot of moisture to the cake/cupcake/slice/muffin, it won't be overly sweet. Plus, you can substitute some white flour for brown (wholemeal or rye) and some white sugar for brown (just keep the white portions bigger than the dark).
What about banana loaf? My favourite! You'll need:
3 oz butter / margarine, soft and creamy
4 oz sugar (try the white and brown mix)
8 oz flour (white and brown)
2 teaspoons of baking powder
An egg, lightly beaten
2 - 3 medium bananas (brown or black old ones are great)
Maybe some spice (a teaspoon of nutmeg or cinnamon)
Maybe some nuts (walnuts, pecans, hazlenuts)
Mix butter and sugar together, sift in dry ingredients. Add egg and mushed up bananas. Pour into a greased loaf tin and bake at 180C until golden and cooked through (insert a knife into the middle, which should come out clean if it is cooked, or push the top with your finger, it should bounce back nicely). Should take a little less than an hour in the oven, but keep an eye on it. Let it cool in the tin for a few minutes, then lift out onto a wire rack to cool completely.
Keeps for a few days, so it is great for lunchboxes. You can eat it on its own, or butter a slice.
I'm going to upload a how to video in the next few days, so check my blogg, www.angiejudge.com. Also, I've got some more banana recipes on there, check http://www.angiejudge.com/essentials-in-the-kitchen/making-use-of-old-bananas.
If bananas aren't your thing (or his), how about ANZAC biscuits? They're very filling, keep for months (literally), so they're fantastic lunchbox fillers and they're not too sweet (plus you can adjust the sugar content easily without too much trouble). Use the 'old fashioned' recipe which has less sugar http://www.angiejudge.com/archives/113. And if he really likes chocolate, you can spread chocolate icing on the top or melt a chocolate button on each during cooking.
Enjoy!