hello anna welcome to the forum!!!
http://www.ukba.homeoffice.gov.uk/po...mesexpartners/
Its complicated and its hard for you to prove and apply for unmarried partner,as you dont plan to have a civil partnership once you are here in the UK or plan to settle due to pre-nuptial issues...you are not on the umarried,same-sex category,..and I assume you have not lived together in a relationship akin to marriage or civil partnership subsisted for 2years or more.
The only solution I can see is for your bf to arrange a pre nuptial agreement outside and inside uk,its not yet implemented on uk law but it will be in the future.. the agreements should be notarized to make it legal so there will be no worries on your bf side.
Good luck!!
