The other way that your body gets vitamin D is to actually make it. The ultraviolet light from the sun goes through the skin and helps to produce a form of vitamin D in your body. Children who don't get enough sun exposure don't make much vitamin D and can become deficient. This is generally not a concern except in the extreme northern latitudes (Alaska, northern Canada, extremely northern Europe) where there are only a few hours of daylight in the winter. The exception to this is that in children who have dark skin (medium brown and darker), the pigment in the skin prevents much of the ultraviolet light from going through, and thus they don't make as much vitamin D as a fair skinned child would with the same sun exposure.
As you mentioned, the combination of no vitamin D intake, and no sun exposure can make a child significantly deficient in vitamin D such that they get rickets. Rickets is a bone disease in which the calcium and phosphorus in the bones get washed out, making the bones weak and subject to deformity and easy breakage. Fortunately, this is rarely seen nowadays.
To answer your question, only about 10 to 15 minutes a day of sun exposure is all most white or light skinned children need in order to produce enough vitamin D. In darker skinned children, probably twice this amount is sufficient. Remember that even on cloudy days some ultraviolet light still gets through; it does not have to be "sunny."
So if your child isn't taking any milk at all, and doesn't average this amount of sunlight, you do need to consider supplementing his diet with vitamin D. It is hard to get this from foods, as the major sources are fish liver, salmon, mackerel, sardines, and egg yolks--things that many young children don't eat readily! There are vitamin D drops that your physician can prescribe to serve as an extra source of vitamin D. Keep in mind that all infant formulas have vitamin D added, so the only babies who would need to be supplemented are breast fed babies, as breast milk does not have sufficient levels of vitamin D (this is NOT a reason to stop breast feeding!).