Attack cat joining sweet cat household

My boyfriend is moving in with me. I have two of the most wonderful cats ever: a boy and a girl (brother and sister) who I save from the street when they were barely 1 month old. I nurtured them, had them fixed, and they are now almost 5 years old and will let you do whatever you want to them. You can pick them up, rub their bellies, play with their paws, I clip their claws, they never hurt anyone and they are perfect angels.
   
But my boyfriend has a male cat named Bobby. We think he's about the same age, but he was found about 3 years ago in an abandoned loft where a bunch of artists lived. He sleeps with us and loves us and always wants to be on our laps and play with us. But out of nowhere he suddenly decides to violently latch his teeth onto our forearms. His eyes get extremely dilated and he goes crazy. Sometimes he comes out of nowhere and stares at the one of our forearms like its evil and then won't stop attacking until we rip him off and toss him in another room. Sometimes while we’re all sleeping he decides to attack as well. There is usually nothing that triggers this, but other times it's when he is in love mode and wants to be pet and then you get your forearm in view and he goes crazy. I've even had to hide my arms under the blankets and he will try to get to them.
   
It is the most painful cat attack ever. I don't know if he was abused or if someone gave him drugs and maybe he is tripping out. We have only introduced him to two other girl cats before (not mine) and one dog. He was super curious about them and paced around trying to check them out but never tried to attack them.
   
My question is this: what could be causing Bobby to attack us like this and also, how concerned should I be for the welfare of my cats once they get introduced and start sharing a household? (side note: Bobby and b-friend are moving into my place so the territory is currently my cats.)

Has Buddy been fixed? If not, do it now.
   
When is your boyfriend moving in? How much time do you have to plan for this?
   
This sounds like a form of cat brainstorm. Sometimes happy cats being petted can become overstimulated & freak out. This, combined with a cat's natural tendency to want to chase & attack something (anything!), is resulting in him having a kitty-freak & lunging for your arms (our Faro constantly attacks whatever is moving under the blanket). Night attacks are also quite common - just ask my husband, the constant victim of Faro's 2am boredom.
   
Also, cats who are alone a lot of the day always have an over-reaction when their people get home. There was no one to play with for 8 or 9 hours, so all the pent-up energy blows out of him all at once. It's quite possible that the problem may lessen when Buddy & Boyfriend move in, because he'll have other cats to interact with all day.
   
Does Buddy have a cat tree? If not, get him one; you can buy or build one (search "cat plans tree OR furniture OR tower"); I build a cat tree from plans I bought online. A good website for ideas: "www.toolcrib.com/blog/2009/01/13/20-free-cat-furniture-plans-free-plans-for-cat-trees-condos-scratching-posts-and-more/"
   
Also get toys for him to chase: stuffed mice - soft balls - string toys that attach to doorknobs - those wooden mazes that you hide a little toy in that the cat has to dig out - wine & champagne corks are often popular (the cat can bite down on them) - battery-powered toys like a Thing In A Bag or a unit that has an electric eye that makes mice pop out when the cat passes by (sources: Amazon, Drs. Foster & Smith, CareALotPets). Throw his toys across the room for him, it's quite likely he'll chase them.
   
It is also possible that his time in the artists' loft may have some side effects, especially if the cat was subjected to a lot of marijuana smoke. And yes, he could be on a trip, if he got into some LSD. But personally, I think it's Brainstorms.
   
Is he breaking the skin when he latches on? Or is it a lot of pressure that only causes blood when you try to remove him?
   
Pay close attention to Buddy at all times, to how he's acting. All cats will telegraph a mood like this by some action or look. Something will indicate it: changes in his eyes, ears laying back or flicking back & forth, rear end wiggling, something. That's when you go into action: Stop whatever you're doing. Grab him by the scruff of the neck & hold him down until he calms down. You can also spray him in the face at these times with an aerosol cat tranquilizer (check your local pet store) or just water with a strong peppermint odor (cats don't like peppermint). Raise your voice & tell him to stop. Even blow a whistle at him to cut through the nut-case mood & startle him into (probably temporary) stillness. Then lift him by the scruff of the neck & deposit him on the floor - and throw a stuffed mouse down the hall for him to chase.
   
If he gets to clamping down, get him by the scruff of the neck & insert a finger at the corner of his mouth, behind his teeth, & pry his mouth open. It's not easy, but it can be done.
   
I also suggest that you introduce him to your cats before he moves in. Take him over to your house to meet your cats in a controlled manner, & let him look around. The visits should be at least every couple of days until the move happens. See how things go. You won't know what kind of personality conflicts you'll have until you have them living together for a continuing period - but that, of course, is a bridge you'll be crossing when you come to it.