My roommates boyfriend will not sign the lease to the house we are moving into. What should I do?

Okay so here is the situation in a nutshell. My roommate and I are moving out of our 2 bedroom apartment and into a 3 bedroom house with her boyfriend. We all go meet with the landlord to sign the lease and drop off the security deposit and once we get there my roommates boyfriend says he isn't signing the lease and "forgot" to mention this to us earlier. He said he isn't willing to compromise his future by signing a lease with someone he is dating in case they break up. So my question is, what should I do about this? Right now me and my roommate are the only two signers on a 3 bedroom house. I was thinking I could make up a roommate agreement that would require my roommates boyfriend to give 30 days notice if hes leaving and outline the amount of rent he is obligated to pay, that way hes not on the lease for the full amount and might be more willing to sign. I'm just wondering what I should be doing?