Parents in multilingual families face the choice of teaching their child both of their languages from the start or wait with the next language until the first is established and the child is fluent in it.
The decision whether to go for one language at a time (also known as subsequential bilingualism) or chose to speak both since the birth of your son (also known as simultaneous bilingualism) depends on a few factors. The things to take into consideration are how much exposure your son is going to get in each of the languages and whether one of them is a majority language.
If there is a chance that he will not get enough exposure to the minority language outside the home, then you should consider only speaking the minority language to give him a solid foundation in it. If there will be a good amount of support for the minority language, then you can well start teaching both languages at once.
The decision of course depends on what languages each parent knows and what language pattern is viable in the family. Each family is unique and will have to adapt any suggested approach according to their circumstances.
There is however one option that is not recommended if you want your son to become bilingual: teaching him the majority language first and bringing in the minority language later. The reason why this is not as good an option is that experience has shown that it requires a lot more conscious effort from the minority language parent. The likelihood that the majority language later on becomes the sole active language for your son is significantly greater.
Does this mean that if you didn’t start teaching him your language since he was born, there is no point trying later? Of course not. You can start at any time, you just need to be prepared to do a bit more planning and work a little harder. Also, remember that even a passive language knowledge will benefit him.
May the peace and power be with you.