I am British, my husband is Spanish and we live in Spain. We have two boys (aged 5 and 3) who go to a Spanish speaking school. Since they were babies, I have always spoken English and my husband in Spanish, which seems to work very well as they both understand and speak both languages.
The oldest is now starting to read in Spanish at school. My question is when should start getting him to read/write in English. Should it be in parallel with the Spanish or should I let him master one language first followed by the other. In Spanish the reading is much easier as it is more phonetic and I don’t want to confuse him.
Learning to read and write is a specific skill that serves specific needs, so the short answer to your question is that your son should be introduced to English literacy when he has the need for it. This could be (most certainly will be!) for schooling purposes, and his need for it is the only timing that matters: it makes no difference to become literate in one language and then another, or in several at the same time.
Having said that, it is almost certain too that your boy will become interested in printed language in general, because of the new printed world that is opening for him through Spanish literacy. He is likely to start asking questions about English spelling, and likely to want to read English texts. If he does, just help him along whatever he asks and whatever he wants to do with his reading and writing. There is no reason for you to engage in formal teaching of English spelling just because he is being formally taught the spelling of another language.
Answering his questions will satisfy a need of his own. Your boy won’t get confused: he already knows that English and Spanish sound differently and are used in different ways, so he will naturally expect that the printed forms of the two languages will be different too.
Do feel free to contact me privately, if you wish to discuss these matters in greater detail.