I'm French and DH English and we live in the UK. DS has been raised as OPOL and is fine with it and has no issues from switching languages depending on whom he speaks to. Soon, a new DC will be born and We will do OPOL again.
However I wonder which language DS should speak to his sibbling? English as it is the majority language or a mix of French and English?
DD1 spoke my language to the baby as she thought it was the only language the baby understood. So unless your DH is going to take sole care of the new baby, I suspect your DS will speak only french (like you!) to the baby. At least at the beginning, when they both grow older I suspect only english will be spoken unless you move to france.