Hi, welcome to the forum!
The best thing to do is to take your cat to a vet as soon as you can. What you are describing could be a skin condition, a parasite, or it could be an anxiety symptom.
You need a diagnosis before we could recommend anything helpful.
Come back and let us know what happens and we can let you know some supplements and dietary regimens that could help, along with whatever medication the vet prescribes.
BTW, is there anything unusual about the skin other than the missing fur - like scabs, flaking, anything like that?
Keep us posted, ok?
My daughters cat had the same,she started feeding him on cat food that only had fish in it,took a long time but the fur is growing back.Are you sure it's not flees or mites.Best get him checked by the vets first.