Have you tried the picnic option - for some silly reason my DS is much happier to try new things when he is either at the park or at someone elses house or at a cafe.

We try and have a "you must try it before you tell me you don't like it" rule.
If he tries it and doesn't like it then he can have something else (but not a favourite thing, just an OK thing). No substitute until things get tried.

Gigi - for the constipation, have you tried things like sultana bran buds, or the tiny weetbix - as "biscuits". They're probably not as healthy as fruit, but they do contain a lot of fiber.