Well, now...........not a very smart move on the part of your "nan" just inviting you and not your BF..........was she trying to cause trouble?
Hasn't "nan" got friends of her own age to accompany her on holiday?
If "nan" had been smart, she would have invited you both and the BF would have said."Going on holiday with your "nan" not bloody likely and there you would have had your get out.
You have only been together for 2 years so BF is feeling, quite naturally a little insecure.
Would you have minded if BF was invited, NOT you, to go away with HIS "nan.?
I would suggest that you haven't thought this out.
P.S There is absolutely nothing wrong in having a "control freak" as a BF....some women nee them.