Your question is whether to start with OPOL now, or wait.  From your writing it is not clear to me whether your wife is actually an English native speaker.  If she is a native English speaker, I would recommend to immediately start OPOL.  There is no reason to postpone this since you have decided on your approach.  Delaying the switch in language does not seem to have any advantages (especially since you indicate he is well exposed to French, and in more ways than just speaking to the parents) and might be more confusing for your son as he gets older.

If she is not a native English speaker (and is a native French speaker instead), I would recommend not to follow OPOL at all and follow MLAH instead (minority language at home, where both of you would speak French at home).  In this case I would also recommend to immediately expose your child to English, through playdates, playgroups, and from 2 year old onward even TV.

My recommendations are basis my own experience: I was raised in a bilingual home, where my mother spoke a language with us children that was not her native language (Dutch instead of her native French).  It affected my development of Dutch in some ways, and heavily underdeveloped my French.  She also regrets it with hindsight.  Currently due to our circumstances of managing three languages, my wife and I are raising our children in a mixture of OPOL and MLAH.  Our experience is that the children understand us best in our respective native languages.