The situation with my new cat is getting worse

Good morning

A little over a week ago I adopted a cat who was in a shelter (he is 8 months old)

I live in a large house in the countryside, and I already had 2 cats and 1 dog, all with serious liabilities. In less than 24 hours my previous animals felt safe and confident in my home.

But this time I came across a shy cat. The first day I could see him but he was hiding in corners. Then after 3 days he started walking around at night, eating at night and using the litter box. But I couldn't approach him otherwise he would leave.

And for the past 2 days, he has started to become aggressive, wanting to scratch if I try to get my hand near. But the worst thing is that he stopped doing his business in the litter box, and started doing it absolutely everywhere in the house. His behavior deteriorates. He has quality food, peace and quiet, his own space, I only ask him 2-3 times a day to talk to him calmly. He is not sensitive to games or treats

What to do?