L.M.
Without using something w/ a fliptop, you will have to go with a thermos (which he may need some help with). If he's going to kindergarten, he should be able to put a straw in a juicebox. If he can't right now, start working with him...show him how, let him try, have him start doing it.
Capri Sun's are great because you can get juice, flavored water, or even the Berry flavored vegetable juice...light weight and you can freeze over night and they will thaw in time for lunch (eliminates the need for an ice pack while keeping lunch cold).