My guess is that he's wanting some paint he KNOWS contains lead to use as a control for checking lead paint detector kits, and finding out which one(s) are the most reliable.

PS:
1. The reason that the different colourants in the paint tinting machine at your local hardware store use glycerine as the carrier fluid is because it's highly soluble in both water and mineral spirits. So, by suspending the different coloured pigments in glycerine, and using those to tint paints, the hardware store can tint both oil based and latex paints (or, all of the paint it sells) on the same paint tinting machine using the same colourants.

2. You should be able to buy food grade glycerine, which is 99.7 percent pure over the counter at any drug store. People spread it on their skin and lips to to keep them moist and prevent them from drying out. Glycerine is a common food additive, so there's no danger in putting it on your lips or even drinking it.

3. Glycerine has the consistancy of a light oil like WD-40, but it evaporates completely without leaving a residue. So, if you need a light oil to help with assembling tightly fitting parts, but want that oil to disappear once the parts have been assembled, use glycerine. It'll behave just like an oil during assembly, but then evaporate completely without leaving any residue behind.
A perfect example of where Dad might want to use glycerine is when replacing the rubber handle bar grips on Junior's bike. A thin film of glycerine between the rubber and the steel will ensure the rubber grips slide on easily. But, as the glycerine evaporates (cuz of the microscopic surface roughness of both the rubber and chrome plated steel) over the next hours, days to months (depending on temperature) the rubber will grip the dry steel to ensure the grips stay securely in place. Once you start using glycerine, you'll realize how handy it is to have around. I use it most commonly for lubricating the inside of paint roller sleeves before sliding them onto paint roller frame cages. It makes the sleeve easier to remove when they're all slimy with paint at the end of the job.