Welcome to MAP. I'm sorry your friend is missing... Do you know where he crossed over into Mexico? and has he been reported missing to the local police, and the FBI office closest to where he crossed over, e.g., Brownsville, Harlingen, McAllen, Laredo, etc.? Another place to file a report is at the US Consulate in Matamoros, Mexico and here is the URL: http://matamoros.usconsulate.gov/
Also, any information you can post here about timelines, pix, details of travel, etc., will help. Many of us are still waiting for information about our missing friends and loved ones, but the more agencies and reports you can file, the better chance we all have of getting the answers.