Given the fact tthat he's taking a relatively low dose, the amount of withdrawal he will experience should be minimal. You're right though, it would have been better to gradually reduce the dosage over time.
Many of these doctors don't have much consideration for what their patients are going through, and they can do pretty much whatever they want. It's clear from the amount he's taking that abuse is not the issue here. Probably be a good idea to see a different one.
Take Care
yes he will probably have withdrawls and yes a doctor can do this. the hydrocodon is what will cause the withdrawls. I'm sorry your son is in pain. what is causing it?