Consider the following, explain that you find the situation shocking and ask him if he would agree to go to Relate. Even if he doesn't want to explore the issues in order to save the relationship, it will at least help you to part in a reasonable way which hopefully will help you to co parent effectively despite separately in the future.
It would be great if he agreed to go to Relate, but even if he doesn't it can make you some good to go in your own.
If he goes, it may help you both to understand what is the real reason why he feels the need to leave, and hopefully you both can work together in sorting that.
And also, tell him things will change in the future, most people who divorce, do when the children are very young, which is when mum is stressed dealing with multiple demands of the children and thinking she needs more help, while dad may be feeling a bit left to the side. It is normal, and it will change, but don't underestimate the huge amount of pressure someone could feel when the first child is still demanding a huge amount of attention and there is another baby coming soon, more much so if you had been put through the stress, and the difficult to ignore hormonal challenge caused by fertility treatments.