Its nearly impossible for one parent to get sole custody, he would have to prove that you are unfit to be in the child's life. Most of the time one parent has "residential" custody, but in a good custody agreement, both parents have equal say as to any big decisions in the child's life, and the non-residential parent has liberal visitation.
My husband and his ex (also never married) have joint custody but live an hour apart. My step-daughter lives mostly with his ex, but is with us every other full week in the summer and every other weekend during the school year. Holidays are alternated every other year except Christmas which is split in the middle of the day. I konw other people who live closer and split the time 50/50 all year - in those families the kids go between houses on wednesday every week.
My best advise is a mediator if you think that there's any way to work this out outside of the courts - they can help you draft a good custody agreement. Avoid courts at all cost if you can, but if there is no way a mediator will work (it wouldn't have worked for my husband) find a good lawyer! Your ex may not be able to get full custody, but if he's anything like my husband's ex, he will play games with the amount of custody you both have.
A solid written legal custody agreement is the best thing you can have! The custody agreement should be really specific and say what the schedule is (who has your son when) and who your son spends holidays and birthday's with. Then everything is set in stone and he won't be able to manipulate the situation very easily. The most important thing is stability for your son... he's young like my step-daughter was when their breakup happened, which is a blessing in disguise because this lifestyle is all she remembers. But they still pick up on the tension and fighting between their parents at any age. The most important thing is to NEVER talk bad about his father in front of him, no matter how angry you are.
Good luck! I know how frustrating it can be dealing with custody! if you need any more information or to talk please feel free to message me :)