Although my "secondary" cohabitates with us he is in a somewhat similar position as your BF in that I already have a "primary" ( my husband). He is not necessarily looking for a wife/children but I don't think would be averse to sharing a (separate) home/life/future/finances with someone else.
He says that any future woman would have to be okay with his relationship with me - I do point out that this might severely limit his field. I recognize (although I am not sure that he does) that our relationship would, in fact, change if he had a "primary" type relationship with another girl. Best case that I see, we build him a separate household on our property, so that they can have their own space together. If she were interested in either myself or my husband as playmates or secondary partners that would be awesome (although by no means required).
Another option for him would be to hang around for a "co-primary" type relationship with me and then have "secondary" or other "co-primary" relationships of his own. Depending on how that pans out (in terms of how we all get along) we may want to go with a separate structure vs. an additional suite on our existing home.
(The same would be true of MrS - however, I see this as less likely, as in 20 years together, he has only ever seemed to be interested in being an "on-call" accessory to my relations with other women if they requested it, although perfectly happy to pursue a platonic friendship or cordial acquaintanceship if that is what they preferred...but things could change.)
Me: poly bi female, in an "open-but-not-looking" Vee-plus with -
MrS: hetero polyflexible male, live-in husband (23+ yrs)
Dude: hetero poly male, live-in boyfriend (4+ yrs) and MrS's best friend
Lotus: poly bi married female, "it's complicated" relationships with Dude/JaneQ/MrS (1+ years)
+ "others" = FBs, FWBs, lover-friends, platonic G/BFs, boytoys, etc.
My poly blogs here:
The Journey of JaneQSmythe
The Notebook of JaneQSmythe