Whoa there! There is no practical limit in how many times plastic headlight lenses can be refinished and made to look and perform like new again. I've refinished the lenses on my 2000 LS400 three times since I bought it in 2003 - once after someone rammed and badly scraped the right lens with a shopping cart.
Each time the lenses came out looking and performing like new or so close to like new that I can't tell the difference.
Tell your son to go to the nearest auto parts store, Walmart, etc. and buy a headlight lens refinishing kit - price should be between $20 and $30. Some kits require using an electric drill and buffing pad - I don't care for these - too easy to get carried away with a mechanical device and grind through a lens.
I've used a Crystal View kit each time I've done it. I've probably more comfortable with the Crystal View kit since it involves hand wet sanding the lenses to remove the crud - similar to wet sanding automotive lacquer between coats. I can exactly gauge my progress with my fingertips. The most important part of the process is a chemical sealer that prevents the lenses from fogging up again or at least slows down the deterioration until the restoration process has to be repeated a year or three down the road.
The following thread has photos from the last time I stored headlight lenses: http://us.lexusownersclub.com/forums/topic/74083-headlight-cover-oxidation/?hl=crystal#entry456058
Refinishing headlight lenses is easy. Now that I've done it a few times, I've got it down to about a 10 minute job.