Yes - the �250 x 20 in cash that you give to your son will strictly speaking fall foul of the associated operations provisions, although whether anyone will discover it is a different matter. If you are reasonably young and in reasonable good health, why not give the �11k now - as a PET (Potentially exempt transfer). If you survive 7 years, there will be no IHT on the lifetime gifts.
�6k is covered in any event with the �3k personal exemption. The IHT on �5k is not going to be worth the candle (about �2k).