I usually keep some bark on top of the substrate to help stop it drying out over time, but you will need to add some water on occasion. Spraying water is fine if only the top half inch or so had dried out, but you will find that spraying alone won't penetrate the substrate very far, you tend to end up with a thin wet layer sitting on top of the dry stuff.
When adding water, you need to make sure it gets deep into the substrate, I like to see some water get down at least one side of the tank and into the lower layers, though be careful not to flood the bottom, particualrly if there are pupal cells down there. If the substrate has dried to quite a depth then the best way to up the moisture level is to remove the larvae and give the substrate a good stir or mix, adding or spraying water as you go until it gets back to the right level of moisture. Dynastes will tolerate substrate at the dry end of the scale for a while but do better in "standard" moist substrates. If you keep them too dry they will shrink over time as they lose water, though they can recover from this if put back into damper conditions.
