Can you take a picture and post it her on the website?
Is he itchy?
Have you left him before like this, does he get stressed?
Do you have any other animals in the house?
I have left him before but not for a while, usually one night here and there. He is quite an itchy scratchy cat, I generally brush him every day when I can as he seems to love it. He doesn't have fleas, I use flea treatment on him monthly, Frontline. I can take a picture but how do I post it to you? Stress wise, I don't think he gets stressed, although I do believe he loves company.
If your cat is itchy, it can be something like ringworm (fungal infection) or allergies. On rare occasion cats will have demodex mange. It is not contagious.
I only got the one picture that is of the lesion itself.
Other possibilities are trauma, like the cat hid under something and scraped those places. Cats can also sometimes lick these places and actually lick the hair off. Since you are home you can watch and see if he was licking at the area or not.
Here is a website that talks about alopecia (missing hair) in cats. The best thing is to have your vet examine him to make sure what the cause is. Ringworm is something that is contagious to people and needs to be treated.