Karen,

It's not that they were abused
I fail to see how leaving a cat out in the bitter cold is NOT abuse.

I'd throw caution to the wind and keep the cat. It's obvious that don't want him. You are an angel for caring for him.

If he's biting you might want to see if he's declawed. So many people have their cats declawed then dump them when they start to bite.

The one thing you might want to do is when he goes to bite, don't pull your hand away. You don't want him to know you're afraid. That is unless he's biting hard. I had a declawed Maine Coon rescue who would bite. When I realized that she was only giving love bites, I didn't pull away. I think if you work with him, he'll get through it.