If he likes sweet wine, .999 isn't sweet. It's, at best, off-dry. Sweet wines are generally 1.020 or so, I think. I like dry wines, but I've made some that I sweetened to 1.000 or so so they wouldn't be bone dry. Dandelion wine, for example, is great at 1.000. It's not sweet, but not puckering dry either at that SG.
As the other poster said, make sure you use potassium metabisulfite (campden or k-meta are other names) and potassium sorbate to stabilize the wine before adding any sweetening. It's easiest to mix it up with the sulfite (1 campden tablet per gallon, or make a sulfite solution and measure it by the teaspoon if you have powdered k-meta) and sorbate (measure by the teaspoon- check the dosage on the package) in water, and add that to the receiving carboy. Rack the wine into it. Let sit a couple of days to ensure that no refermentation will occur. Then, you can sweeten the wine to taste.
The easiest way to sweeten the wine by the batch is to pull out many samples. Then, add your simple sugar syrup to each sample, and rate them as to "not sweet enough, too sweet, just right", etc. When you find the one that's just right, take the SG of that sample. You then want to sweeten to just UNDER that SG. That is, if you like it at 1.012, sweeten the batch to 1.010. It gets sweeter tasting in the bottle.
You'll have to calculate the amount of sugar, but free programs like winecalc help and can tell you how much sugar to add.
Broken Leg Brewery
Giving beer a leg to stand on since 2006