Trying To Get Custody After 15 Months

I (21M) have a 15 month old son with my now ex (21F). As a little background, we lived together with two other roommates when we found out we were having our son. She moved out not too long after and back in with her parents. I’m more or less dead to them since I got her pregnant, as we weren’t on good terms before. Anyways she moved back home for health reasons , and has lived there ever since. When our child was born, I wasn’t notified until he was actually here. The parents confiscated the phone the night before and didn’t tell me when labor began or anything. I was 3 hours away, and by the time I was there, the name was chosen. Every time I asked about the birth certificate, the parents would just assure me it would happen at some point. I came to find out it did before I arrive and no one told me. Me and my ex for the course of the last year have dated lightly but have mainly only seen each other so I can see him, which is next to never due to the parents . The actively prevent me from seeing him any time they can . I usually saw him 2-3 times a month but face timed him every day. I have been trying to get on the birth certificate since day one, but the mother or her parents always seemed to usher it to the side and never let it happen. I grew up in the system after a messy divorce and didn’t want my son to go through any of that, so I’ve tried to avoid it this far. That changed a week ago when she decided not to show up for a time when I was supposed to see him, and has refused to say anything or let me see or talk to him this entire time. Now I’m trying to figure out how do I get on the birth certificate and how do I get custody. We are in Florida. Thank you in advance.