He can have some sugar (and he does on a fairly regular basis) but if he eats a lot his behavior goes haywire and he drives us bananas!! I've found he's OK with pretty much anything that has at least some semblance of nutritional value. Granola bars, even though they have a lot of sugar, also have the whole oats, so he's fine with that. Dried fruit is fine, even though it's very concentrated sugar. Chocolate or cookies, on the other hand, set him off.
Don't get me wrong - he eats his share of chocolate. It's just that I don't want him snacking on it all day long in order to get his calories. I'd rather have him eat nuts.
I'll keep an eye on him and see if he keeps losing weight, but I don't think he will. We've been on the road nearly two months now and I've learned a lot about what kinds of foods and how much food we need so I think we've mostly solved our problems. I'll just put nuts or fig bars in his handlebar bag rather than carrots and that should be enough calories to keep him in the black. And of course, once we get down south we'll slow way down so it won't be an issue at all.