I don't think you can buy for your nephew (or godson) if he is under 16 - I think they can only be purchased by his parents, grandparents, greatgrandparents or guardians. It would also have to be done by post, using one of the National Savings & Investments application forms (not on-line).
Here's the website that explains it all
http://www.nsandi.com/products/pb/howitworks.j sp#investing_for_a_child
You could of course get one of the application forms from the Post Office and hand it to your nephew or his parents along with a cheque. Nice present !!