Best way would be to give your son the money and have him purchase it. But you don't generally have to pay sales tax on a gift. This is not legal or tax advice. If you have questions you should seek a tax professional.
If you have it lisenced in your name then yes you would have to pay to register in his name. I'm not sure what your situation is but you should be Abel yo call them and make sure before you set up an appointment to go in.
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