When my DD's hair was that length I would spritz it on non-wash days with either glycerine mixed with water, or Luster's No Drip S-Curl. We still use the latter product regularly in between washes (her hair is now waist length and usually braided). The name and packaging are horribly 70's but the ingredients are simple and good (mostly glycerine, no oils or grease) and it leaves a nice shine while encouraging curl. We've always just used conditioner to wash--no shampoo. Works well with this product.
I recently left her hair loose for a week and we used the Luster's every day to boost curl (this reduces frizz for her, by making the hair form ringlets). Not sure if this would work as she naturally has very tight, shrinky ringlets, and his curl looks a bit looser, but might be worth a try as it's cheap and comes in a spray bottle so it's easy to use
I should mention that this technique works best when there's humidity in the air.