To keep or not to keep...

That is the question. So my husband and I took in a stray about 6 months ago. We already had 6 cats, but we couldn't bear for the little ball of fur to be all alone out in the elements, so after spending a few weeks feeding him and bonding with him we brought him inside. 6 months later he still growls and hisses every time he sees one of our other cats. He isolates himself in our spare bedroom so aside from feeding time, he doesn't get much human attention either. I feel like he would be much happier in a single cat household, but I don't know anyone who would take him and I really really don't want to give him to the pound. Any tips on acclimating him better? Or tips on finding him a better home for his personality?

I would suggest setting him up in one of those nice framed fabric crates.  (I keep two around, they are collapsible and the fabric can be removed and washed).  You can put the crate in a corner in some room where there is cat and human activity.  At first cover it with a sheet except on one end so he will feel safe and hidden.  As he gets used to everything, then you can pull the sheet further back.  I use these crates with feral cats and kittens and it does a lot to get them acclimated.  As long as your feral guy is growing up in isolation, he will never learn to be a family member.  If he never adjusts (and some of them don't), you can let him be outside again after he is vaccinated and neutered but hopefully he will get used to the household, especially if your other cats don't bother him in the crate and just let him be.

Chewy.com has a good selection and they have free shipping.  Go with the large or extra large dog size.

https://www.chewy.com/b/crates-kenn...y06znPtBkI4bcMpIRsqt4uFObMTrHvwc4kaAmMo8P8HAQ

Thanks for the advice Red Top Rescue. I have a very large dog crate. If I put a blank and pillow inside and cover if with a sheet for added privacy would that work? Should I maybe put a food/water dish inside and shut the door so he can see what's going on, but none of the other cats could bother him? I can pretty much guarantee if he has a way to escape he will immediately flee back to "his" room. I don't want to traumatize him, which is why I haven't pushed him thus far, but it's been months now and he's just not adjusting. He also has no interest in going outside. We have a doggy door leading to our yard which I leave open when the weather is nice so our many animals can come and go as they wish, but Poofy (the stray) shows zero interest. He seems content to spend all his time in his room, but I feel so bad for him. Being alone all the time can't give him much quality of life. If he doesn't adjust should I try to find him a new home where he would be the only cat or is he okay living a relatively solitary life?

Living a solitary life is fine, but not if he's scared all the time.  Being scared is stressful.  You might start the dog crate thing at night, when there's nt much at all going on, and he can start with quiet and dark.  The crate will eventually become his safe place, the place he will run to if scared.