Get the exact wording from him - Customs charges are usually the responsibility of the buyer. In fact ask to see a photo of the letter. There's actually no way for you to pre-pay customs charges delivering to the UK.
Basically, if the item (alone) is valued at more than £15, he will be charged import duty+ VAT (sales tax) on the item AND postage costs. There's also an extortionate "handling fee" of £8 that the Post Office charges for collecting.
If it is Customs, he can pay it and get his item (it will be sitting at his local Post Office), or not pay it and not get it. I wouldn't hold much hope of arguing the case - the police wish they had the powers of Her Majesty's Revenue and Customs...
I recently paid £18 customs on £50 worth of imported goods - it is getting expensive.
Underpaid shipping would be surprising.