No real problem if your BF has the responsibility to properly care for what he has. Rodents are really great little pets, but they do need some socializing time with their keeper.
At one point in my day, I had 5 mice, two ratties and a trio of gerbils, and well taken care of. Even vet if it were needed.
I would just say it is up to your BFs responsibility, numbers don't mean much.
If his pets take away your time together, perhaps let him know, he may not be aware.