Found my kitty but he's skinny

My 4-yr old neutered kitty went missing for 12 days. He finally came home but he's lost weight and he's not as full of energy as he used to be.
He's been resting a lot and I've kept him inside (hard to decide whether I'll ever let him out again at this point) but I'm worried about his health. Does anybody have any advice about getting him back in shape after this ordeal? How long should I wait to flea-treat him?

Take him to the vet immediately.

so pleased that he came home

I agree - take him to the vet straight away to get a full health check on him

Get him to the vet ASAP as Hissy said.
In 12 days He has come in contact with other cats.
You want to be sure that he has not contracted some diesease.
Cats are great at hiding their illness. Just cause he only looks skinny, doenst mean there isnt anything else wrong.
The Lethargy you describe is not a good sign.

But most importantly.
Im happy he came home to you safe.
So many people never know what happened to their loving furrballs that left home.

Any update on your cat?? I hope that you were able to take him to the vet, in case he' d been injured; he could have been to hurt to come home any sooner. Again, he might check out okay, but please, don't take any chances!

I'm so pleased your cat has come home. I have to agree with others, take him to the vet ASAP. Hopefully he will be fine and just be a little tired and hungry but it is not worth assuming thats all it is incase he has something more serious wrong with him.
Let us know how it goes. **hugs** to you.