Charges by correos

Yes, if sent from outside the EU. Will be made up of Duty on goods, based on declared or assesed value and a Correos handling fee.

The same applies to the UK when buy goods from overseas, including the EU.

There is a low threshold goods value that does not attract a charge, just not sure what it is, sometimes seems the post handlers don't either.

I now tend to buy from amazon.co.uk for gifts to family in UK.

I use amazon.es and/or other spanish companies for our own purchases.

Yes, its a result of Brexit. In future get him to buy things on Amazon. Lots of people are being charged to receive packages due to import charges. 

This link explains what a sender should do...https://www.postoffice.co.uk/mail/customs-forms/CN22-form-guide