Both points of view can be true. Some cats DO need to be "only cats" but some cats just need more time to adjust.
1) How long has Boo been inside?
2) how long before the first meeting of one of your resident cats and Boo?
3) How long ago was he neutered?
4) How old do you suspect him to be?

Now here's why: The older he is the more set he will be in his ways. Therefore, the longer it may take to settle in to a new routine.

Some cats need WEEKS if not MONTHS before they can be left alone with another cat that is strange to them.

Hormones will take a while to dissipate in the body system, so a male cat that at one time may have been a dominant cat, will need time for the feelings to wear off.


And last but not least, find him a new home if you must, but don't be hard on yourself. Just because THIS cat was not a match made in heaven, doesn't mean that there isn't another one down the road that will fit right in.

One more question: Did he draw blood with claws or teeth? Many a time a paw with claws that are too long accidentally draws blood. It may have been more accident than viciousness.

Keep us updated.
Good luck