Same here Mark. I'd go without more if I felt another cat could be introduced to the group successfully but I'm not sure any of the cats would benefit; the existing group or the newbie.
Sage is really nervous and Darwin and Astryd are for the most part quite happy with each others company. But even Astryd, who is one of the most patient cats I've ever met, gets pee'd off with the Little Guy from time to time. I'm not sure how easily another feline personality would fit into the mix.....
Having said that, with the right temperaments, large groups can definitely work. Carrie, Jack, Toby, Parsley and Sage all lived well together and there were never any hisses or growls to each other.
I'm sure your OH is as aware of this as you, and just wishes he could help Tommy.