Try a good hepa filter and keep the ferrets in one room while he visits.
 
Try wiping the ferrets down daily with a wet wash cloth to get the dander
off of them.
 
Change the bedding before your boyfriend comes over
 
Bath the ferrets in  a cat allergy shamoo
 
Try using vacuum cleaner bags with filters on them or buy a hepa
filtration vacuum cleaner for vacuuming before he comes over
 
Try using anti allergy carpet shampoo when you steam clean the carpets.
(For me the actual anti allergy shampoo makes me sick.)
 
I hope some of these ideas help.  Take what you need and leave the rest.
You'll get lots of good ideas from other people.  check the fml archives.
They are a treasure trove of info.
 
KG
[Posted in FML issue 3881]