A quail's egg is the size of a normal egg. I don't think that your doctor was serious when he said this.
0.5cm is much smaller than a quail's egg. 0.5m is roughly half the size of your little finger nail lengthwise. This is still quite large and even if it breaks up as it comes down it can still cause problems with pain, nausea, vomiting, diarrhea, and infections in the urinary tract as the stone starts to travel down.
A stone can be the size of a grain of sand, but can still cause lots of problems.
Your father's urologist may decide to shatter the stone and let it come out on its own, or may operate to remove the stone, depending where the stone is.
Tell your father that he must drink lots of water - 2 litres a day is usually recommended, but his doctor or urologist may have told him to drink 3 litres or a little more. Your father can also try drinking water with fresh lemon juice to help break down the stone. I do not know if this works or not, but some people reckon that it does. I know that when I have had a problem (I am prone to getting kidney stones), when I start to drink lots of water with freshly squeezed lemon juice everyday, it comes away like a sandy sediment in the urine.
The doctor can prescribe stronger pain relief medication than what you can buy over the counter.
If your father does start to experience any of the symptoms I mentioned above and notices that his urine is dark, cloudy and has a very strong smell, he should get the doctor to examine the urine for any infections. If there is an infection, the doctor would prescribe an antibiotic.