Ok---he is making a grape wine here. Not something that is a kit or early drinking wine. Therefore, all these wines need to spend time in bulk aging, and I suggest a minimum of 1 year and most likely 1 1/2 years before any backsweetening attempts. There are a number of reasons for this:
First, you cannot add sorbate to cloudy wine that is full of yeast cells. It is very important to remove as many yeast cells as possible so that the sorbate can work to prevent the few remaining yeast cells from causing a possible re-ferment.
Second, you can't evaluate a young wine for any level of sweetness because the whole attempt is not just making a wine sweet. Rather, it is more of a balancing of sweetness with the acids present. And when wine is young and not at its robust flavor, it is very hard to evaluate this. Additionally, if you have a delicate wine, it's even more difficult to evaluate it and you will probably end up sweetening it too much and THEN the flavor is very tamped down. You have to be careful when sweetening delicate flavors.
Third, bulk aging does many things for a wine. You have to allow enough time for sediments to flocculate together so they fall out. And there are more bulky types of sediment that you can see, but there are more micro-sediments that should be allowed to fall out. Such as volatile esters, as they are unstable components as well. Many of these esters are microbial in origin and not considered to be part of the varietal characteristic. Bitter and astringent compounds can also be removed thru bulk aging--BUT, you have to allow enough time!! All these types of components are unstable----so aging can be thought of as stabilizing the wine once these unstable components have been allowed the TIME to fall free of the wine.
Fourth, aging stabilizes tannins,pigments, and color.
So----you are doing yourself a disservice by hurrying your wines into the bottle.
Bernard---sorbating and adding of meta is done right along with sweetening a stable and properly aged wine. We do it at bottling time and immediately bottle. However, don't add sorbate directly to the wine because it is not soluable in wine. It is only soluable in water where it becomes sorbic acid which IS soluble in wine.