According to family law, what's best for your son is to maintain a meaningful relationship with both parents.
It's understandable that, having separated, you don't want him coming and going whenever. There is now a different level of privacy and respect that needs to be adhered to seeing as he doesn't live there anymore.
Sit down with him and write down days and times that will work best for your son. Take note of the schedules already suggested by sammy and allforher. These two are a wealth of information who reflect the reality of these situations extremely accurately.
Seek mediation, write up a parenting plan and if it works, turn it into consent orders. Litigation causes unnecessary friction and will send you into a massive hole of debt which won't make you feel any better regardless of the outcome.
Try to work it out between the two of you, peacefully. For the sake of your son.