Thanks for your enquiry.
You are right in being slightly concerned over the proposed extension. The neighbour should of course not be building over the shared access, so you have 2 options-
1. Write to him, via a Solicitor, threatening injunction proceedings if he does build.
2. If you are happy for him to proceed, which I think you are, you both would need to enter into a Deed, releasing your right of way. This Deed would need to be drafted by his Solicitor and registered at the Land Registry. This would in effect then extinguish your right of way.
To be honest, it is more of a problem to your neighbour if the above is not done- as any potentially Buyer of his property will be advised not to proceed, on the grounds that you could, in theory, request that any extension be removed!
It would therefor be in your neighbour's interest to get the Deed done and for him to pay for both his and your legal fees in executing the Deed.
I hope this assists and sets out the legal position.
If I have helped, please would you be kind enough to rate my answer.