EU law requires all member states to permitt the free movement of goods, including foodstuffs. (The only exceptions are temporary bans on health grounds, such as during the BSE scare). So you take black pudding (or any other foods) into any EU country, including the Netherlands.
I'll add my support to the Eccles cakes (especially if your son can persuade the Dutch hosts to warm them up), although I also like everything else which has been suggested so far.
Biscuits such as Custard Creams seem to be unknown on the continent but, even better, how about sending your son with some good ol' Jammy Dodgers?
Galaxy chocolate is almost certainly unknown in the Netherlands (if for no other reason than the fact that it doesn't meet the European legal definition of 'chocolate').
Gherkins?
Pickled onions?
Bloaters?
Tripe?
Heinz syrup puddings?
Steak & kidney puddings?
Scotch pies? (OK, they're not 'English', but they are 'British')
Scotch eggs? (Ditto?)
(I've got to stop. This is making me hungry!)
Chris