Thanks for the support everyone.

The people who he is with now took him to the vet this morning (yes, it's a boy) and he is healthy. I was told he is eating well and when looking through the window at the animals seemed unfazed. He will be introduced to the dogs and the other cats gradually over the next two weeks and once that two weeks is over he will be let outside. Hopefully, he will then decide to stick around as an indoor/outdoor cat.