Hi Audrey
The winnings from premium bonds are not taxable income.
For Inheritance Tax purposes (not the same as Income Tax) you will have given your son the maximum tax does not count towards Inheritance Tax, before you gift him any of your premium bond winnings.
Any cash gifts over the £3000 'annual exemption' will count towards your estate for Inheritance Tax purposes.
Please have a look ahere:
How Inheritance Tax works: thresholds, rules and allowances
Thank you.