As a Dad who has been through similar circumstances it does not have to be all doom and gloom. I also understand as his mum your instincts are to protect him, and that is perfectly natural.
It is encouraging to hear he wants to be involved with the baby, and that in my view is to be supported, but at this stage the person who needs it the most is the mum. She will be feeling absolutely up against it at the moment, hormones, legitimate worries about the practicalities, health implications. The absolute last thing she needs at this stage is a prospective father acting in an adversarial fashion.
I am purposefully avoiding the rights and wrongs of what went on between them. She could be an absolute devil woman for all I know, and that to an extent can be where you come in. If he needs to rant about something let him sound off to you, as it is far better he has a sympathetic ear than take any angst into a situation where he has to deal with his ex. If his priority is now his unborn child then entering into any conflicts with the ex is utterly fruitless to working towards that goal anyway, and can only harm that outcome.
The reality of the situation is that throughout the pregnancy, and the first few years of this little one's life it is best to think of baby and mum as one in the same, if anything compromises the mum that means it is compromising the baby. I know it might be very difficult, but if the both of you can stay the course, resentments between them WILL fade, and what will be left will hopefully be a lovely happy child securely bonded with both parents with two happy homes.
So basically at this stage hold off on recriminations and negativity, and get him to repeat like a mantra: "supporting the mum is supporting my baby". I cannot emphasise too much how worthwhile this will end up being in the end. There will be many fears, worries and problems you could foresee down the road, but do not be paralysed by them, and try to see the areas where you CAN work towards a positive outcome and at this stage and at this point that is to put a full stop on the arguments that have gone before, and offer to support the mum in whichever way she wishes. By all means communicate an earnest desire to be involved, but for the next little while it HAS to be on mums terms. Honestly early contact can only happen if mum is comfortable, so logically that has to be the only objective right now.
I am speaking from personal experience, that every morsel of resentment I swallowed early on, and every potential argument I wanted to engage in that I forced myself not to allow to occur that whilst it may not have felt like it at the time has been very worthwhile now I am in a decent position to co parent a child I love more than anything in the world, and I know loves me.