Advice please😭custody battle.

I got pregnant at 18, I told my boyfriend at the time and he left me... I tried to have him pay child support, I asked him through text messages and he declined saying "he's not my fucking son so stop texting my phone." and other hurtful things directed towards our son, I still have those messages now because I screenshoted them and made sure to screenshot the number as well just in case. After that I raised my son by myself for 2 and half years, didn't ask his father for shit. But I was a single parent until my boyfriend came into my life and helped me raise my son now. My boyfriend and I are now expecting a girl due in October of this year. Now my sons father reached out to me on Instagram and wants to come back into my sons life, which is okay, as long as he helps take care of his son and has a stable place, we agreed on him seeing his son, but now he's threatening to go for full custody of my son because I'm not fit to be his mother which is total bullshit because my boyfriend and I have our own 2 bedroom apartment and we are looking at houses now to move hopefully in the future. He's really not settle to even have full custody, he doesn't even have a place he's either in the shelter or sleeping from house to house which isn't okay with me. I don't mind us sharing custody but for him to aim for full custody is fucked up. Update to now: He wants me to leave my boyfriend now too so I can go back to him. Saying "we can be a family, just you, me and our son." Like he hasn't even gave a shit about him for the last 2 and a half years. I'm clearly not going to leave my boyfriend for him which made him even angrier with me. But I'm getting a lawyer. I just really need advice on my rights, I'm really new to all of this and ik I shouldn't be stressed while pregnant but it's just really hard not to be.

UPDATE: He's still pushing for full custody, but I'm taking you guys advice because now that I think about it, I really don't need a lawyer. He's too unstable to even get partial custody. The other day he texted me saying how he's getting himself a good lawyer so I might as well kiss our son goodbye because once he has full custody I'm never going to see him again.. he's just such an asshole.