score:4
The question is opinion based as explained in the comments however if you want a rough idea I would put it around 70% probability of success applying some crude statistics.
How do I arrive at that? British citizens have an adjusted refusal rate of 20% on average or 80% approval rate.
British citizens have visa free entry to USA hence those who apply for B visas are generally the small group with some problem on their record (your boyfriends category) or those who want to stay longer than 3 months allowed for visa free entry.
He owns a home, has a career, and seems settled in the UK a prosperous western country, although not married. However his past overstay (although the ban is spent) will have an overhang on his profile.
Thus I would put his odds a little lower than the 80% average and arrive at 70%+/-.