I've just been going frantic in the attic! I know that I had a much-treasured copy of the children's edition of Don Quixote which I really enjoyed when I was your son's age. I've looked everywhere for it - because you could have had it for free - but I've had to conclude that it must have been consigned to the charity shop at some stage.
All is not lost, however. You can buy it secondhand through Amazon UK. Check these links:
http://www.amazon.co.uk/exec/obidos/ASIN/0460050249/qid= 1132708338/sr=1-10/ref=sr_1_10_10/026-6023873-4369259
http://www.amazon.co.uk/exec/obidos/ASIN/B0000CIN9X/qid= 1132709249/sr=1-32/ref=sr_1_2_32/026-6023873-4369259
Chris
PS: It's good to hear that your son has discovered this book. The list of the 'most published' books in the world reads "1. The Bible, 2. The Quotations of Mao Tse Tung (the 'Little Red Book') , 3. Don Quijote de la Mancha, ..." but most people in this country have never even heard of Cervantes! I'm sure that must say something about our education system!