Almost certainly you could force removal of the fascia, but it depends exactly where the boundary is. This type of extension has great potential to go wrong, and potentially cost neighbours thousands of pounds in legal costs - so be very careful.
It's too late to tell you this now, but this work should have been subject to the Party Wall Act 1996, and if you had used it you could have employed a surveyor at your neighbour's expense to ensure no misunderstandings of this type occurred. I mention this now so anyone else reading this remembers it before the time it happens to them.
http://www.communities.gov.uk/publications/pla nningandbuilding/partywall
It is very likely that the outside edge of your wall is not on the boundary because the verge edge on the roof above usually protrudes further by 70mm or so. If that is the case he cannot link the wall together because the verge edge above will be on the boundary. However we are talking a few millimetres here and because he has already gone to the cost of completing the work he is unlikely to want to change things without a fight.
You say the fascias are now linked - please explain the orientation of the two roofs - are they are same height and orientation? Has he joined up at the roofs?
You presumably don't have guttering along this common boundary to him?
The simple rule is that no part of the structure shall protrude into another's land - no guttering, no fascia.
Post some more details and I'll try and help further.