Without knowing it’s about feathers, the procedure I would use would be to point out to your dad that kg is a unit of weight, or mass, and that volume is a measurement of space, which are two different concepts, so the question is meaningless and should begin with understanding the terms it uses. “with an approximation of 10%” is also meaningless, and would be the next term to learn about.

Adding that it’s feathers helps, but as @ragingloli pointed out, feathers do not have a standard density, even for the same type of feathers, so there is no way to get within 10% accuracy.

If the material were something with a much more predictable density, such as water at standard temperature and pressure, then the volume tends to be a simple matter of dividing the mass by the density. Since feathers vary widely in possible density, it’s not possible to be accurate – you’d need some estimated figure for density, which you could get by weighing feathers stuffed into a box of a known volume, then dividing (total weight – box weigh) by box volume to get the density of those feathers in that condition.