You are kind to be so helpful to your neighbor. I do have some suggestions to make this task a bit easier. 

If your neighbor has been accumulating papers for a long time, there’s a good chance that many papers are both financial and confidential in nature. Because you don't want to risk tossing something that is important. I suggest you first call in a seasoned organization pro who can help him with the first level clean up. This would involve both of them going through everything to separate important-looking documents from paperwork, mail and other "stuff" that can be disposed. The National Association of Professional Organizers has a user-friendly directory that can help you locate a professional in your area.

If your neighbor has numerous financial papers, or it appears that bills have gone unpaid, I’d suggest hiring a Daily Money Manager (DMM) who can evaluate what appears to be important paperwork and set up a good filing system.  He or she can also arrange for proper shredding if that's needed. Daily Money Mangers provide an array of financial services including paying bills, keeping track of medical and other insurance papers and organizing your files and home office. Going forward your neighbor may want to have this person come in once a month or more frequently to help manage his finances and maintain the filing system. 

To find a daily money manager close to you, use the "search by proximity" link on the website of the Assocation of Daily Money Managers. 

Finally, to really clean his home once the papers have been sorted and unimportant things discarded, you can either hire a cleaning service, one that routinely does complete background checks on their employees, or hire an individual who has been referred to you or your neighbor. But remember, it's important to have a background check done on anyone that enters his home. In many areas, this can be facilitated at the local police station. I would also interview helpers before they commence work so they earn your own "seal of approval.