Teaching DS to read/write English in a Spanish-speaking country

[PinkSkiesAtNight]

DS is almost 5 and is learning sounds in capital letters in a Spanish-speaking school. They don´t start reading until age 6 here but he is very interested in sounds and writing at the moment. I have some experience of teaching English phonics and so have been practising blending with him, using capital letters. As he is showing so much interest, I would love to take this further with him, but I am unsure on the best approach.

Should I teach him lower case phonics alongside his school learning? Only get him to blend phonetically, using upper case letters, the sounds that are the same in Spanish and English?
Or just stick to helping him in Spanish?
Help him read by word recognition in English?

The other thing is that all our English books, which he loves, are in lower case. Is it possible to get books in English but written in capitals?

He is quite bright and copes well speaking 4 languages, though English is by far his strongest. I would love to encourage him as much as possible, but I don´t want to confuse him. Any advice would be greatly appreciated.