Any advice???

I am currently almost 17 weeks pregnant and i am afraid that it is going to cause me to lose custody of my 8 year old. My husband and i split in march of this year, in may myself and my son moved in with my new boyfriend ( he was a family friend since i was 10, and my son already knew him). I became pregnant in august. I was unable to get divorced due to financial issues (i was left with alot of bills when my husband moved) so we are still legally married. Latley he has been having me put everything in email for his lawyer. I am scared that somehow he is trying to get full custody of our son. I have never done anything wrong ( no criminal history, no drugs and i dont drink) my current boyfriend is a cop and we share 50 50 custody with my sons father at this time. We both are aware we will be getting divorced and having our custody arrangement after i give birth. I have no problem sharing him with his dad, i just want to k ow if legally he can take my son from me because i am having a child with someone else while married! Should i be worried? (Btw i would ask a lawyer, but i just cant afford it right now) i live in missouri, if the state matters!