I think it's a big mistake to put your name on the lease. It should just be in your boyfriend's name. What if the freeloading friend moves in and becomes the houseguest from hell? For example, suppose he doesn't contribute a cent to household expenses and won't even get a job? If your name is on the lease, it makes it more difficult to move out if you can't take the situation anymore. BTW, I'm with you when it comes to houseguests---I can only take so much. I like my privacy and alone time---both hard to get when you have houseguests.
Considering you lost twins--and I'm so sorry for your loss--your boyfriend should be more considerate regarding what you've been going through. Despite this guy being his best friend, your boyfriend right now must put your needs first.