There is nothing stopping you giving anything you like to your son.
Whether amounts above the limit get counted for IHT depends on your honesty, the honesty of your son and of your executors - and the records available to them.
As to small amounts, it's doubtful if they'd get noticed, and if you made a habit of it might come uner the 'normal expenditure' exemption:
Normal expenditure out of income
Gifts which are made purely out of income as part of a person's normal expenditure are exempt from inheritance tax. The claimant must show that after allowing for the gifts the donor was left with sufficient income to maintain their usual standard of living and that there was an established pattern of giving.
Bear in mind that you are going to survive 7 years anyway, aren't you? (Just to look on the bright side.....)