need advice about Joey

Hi,  I have cats.  Jack (the oldest and my best buddy), Morgan (the handsome gentleman), Jerry (the comedian) and my sisters cat  Mike (the bully).  I don't know how or why, but word has gotten out that I have a fish pond, bird feeder and woods next to my yard so any cat that needs a home show up here!

The newest arrival is Joey, a true feral.  He started coming around in April.  Its taken months but now he comes when called, eats while I sit next to him, loves to be petted, and has actually come in the house a few times to eat.  My problem is the other cats.  Mine are getting used to him (finally), but Mike will attack Joey for no reason.  

I'm worried about Joey in the winter.  I made him a wonderful shelter but so far he doesn't seem to be interested in it.  If I could get him to come in the house and be comfortable around the other cats (especially Mike) I'd feel much better.  Any ideas?

Re the shelter he isnt interested in.  Does it have an emergency exit too?   It should have an escape route backwards.  So the cat doesnt risk to be cornered inside by a vily enemy.

With an back door such a shelter becomes immediately more interesting for an ferale.

Now, re socializing.  You seems to be on a good way to socialize him, and make him to accept the indoors.  You are de facto a good half way mark, not less and perhaps more.

Its Mike who is the problem, and the solution needs some analyzing...   Is Joey neutered?   Mike?

Thanks for the tip about the back door.  I'll definitely give it a try.  Mike is neutered, Joey is not.

Definitely get Joey neutered as soon as possible. Mike is threatened by the newcomer - especially one who is intake.

If you have a spare room for Joey alone, you might be able to slowly introduce him to all the cats. If not, the back door on the shelter might be your best approach for now.

Thank you for helping him!