Fortunately, most young children can learn two or more languages as they grow up, especially in the years before they go to school. They might show slight delays in vocabulary growth in each language at first, because they are learning two or more sets of words at once. But by the time they have reached grade five, they often have a more advanced knowledge of language than other children who speak only one language.
When a child is learning two languages, she may mix words from both languages into her sentences, but she will eventually learn to separate the languages correctly.
You should go ahead and speak the language you are comfortable with to your child. It's also good to read to him in that language, and use it when you are playing with him, as well.
And remember, it's much better to speak to your child in your native language often, than to talk very little because you think you should only speak in English or only French, and you aren't comfortable doing so.