OH I understand how you feel!! I always feel like a nervous wreck whenever I go on a vacation due to worrying about my cats so much.
I noticed it really helped me to know that the 4 cats keep each other company while I'm gone so my friend if pressed for time can just run in, feed them, clean the litterbox out and make sure everything's okay.
Since you have ONE cat, I would strongly recommend having someone stay at your place for the week if possible (meaning overnight). If that is not possible, maybe that person can spend a couple hours per day with the cat playing, etc so the cat won't feel so lonely?
Be sure to leave plenty of toys (SAFE toys) out for him to play with. Do u have scratching posts? Cat condos? Shelf for him to watch birds/squirrells outside? That will help also.
One thing I did that helped was to ask two friends take turns tending to my cats so one friend would come and visit 2-3 days then the other friend came the 3-6 days so that way the friends wouldn't feel so pressured having to tend for a week. (Many of my close friends aren't really crazy about cats..)
Don't wash ALL of ur clothes- keep one-two favorite DIRTY clothes/shoes out so ur cat can still smell you..
Buy some Feliway which calms cats down..
Have the ppl who will check on ur cat give him treats and play with him for at least an hour if possible.
Yes, inform ur neighbors that they might hear ur cat crying.(If u could have a friend stay overnite, that might help prevent him from crying?)
I don't recommend letting him out while you're gone- who knows he might take off... (Actually I'd recommend keeping him as an inside cat if possible grin).
From my experiences with leaving my cats, various things happened when I was gone- while I am gone, my cats tend to express their HMMM DISAPPROVAL OF MY VACATION by wrecking my apt so my poor friends have to clean the messes up. (They've broken 2 lamps, littered the apt floor with towels and blankets, dragged clothes off the shelves, vomited several times, toppled some plants, etc..) So be prepared for that!
When I get home from vacations, they do one of the two things- express their JOY of having me home by being all over me and not wanting to leave me for a second OR express their DISAPPROVAL by ignoring me for a day or so!
Same thing might happen to u so if ur cat ignores u for a couple days, don't freak out- it's his way of telling u he isn't happy with u leaving him.