Librivox has free downloads of books which are out of copyright. They will allow anyone who wants to volunteer to be a reader, so the quality is variable and you may not like the accents. Still, free is free and if your son may like such authors as Kipling or HG Wells then it's worth a browse.
Try your library service. As Creature says, some have free downloadable audiobooks. My library service also does free loans of CD/tape audiobooks for people whose disabilities or learning difficulties make it hard for them to access paper books.
Keep an eye on BBC iPlayer for Book of the Week etc.
If your son has a favourite book which he might like to revisit, it could be worth you recording it yourself.
Website with reviews of audiobooks: www.audiofilemagazine.com/
www.listening-books.org.uk/ Subsidised audiobook service for people with an illness or disability who find it difficult to read due to visual, physical or learning difficulties such as dyslexia.
audiobookradio.net/ Free radio station dedicated to broadcasting stories, drama, poetry and interviews
www.calibre.org.uk/ "Calibre Audio Library is a national charity providing a subscription-free service of unabridged audio books for adults and children with sight problems, dyslexia or other disabilities, who cannot read print."