What do yours and your neighbours deeds say? you can get these online from land registry for about 5 or 6 pounds each.
This should tell you who is responsible for the boundries.
If they are shared (which nowadays is more unlikely) then he should have got your goahead or at least informed you. If they are his then out of curtesy he should have informed you but doesn't have to. If it is your responsibility then no he shouldn't.
Either way he would need your permission to come onto your land if he needed it to errect it.