Those of you saying she needs to work more, have you missed the way he behaves, and esp around her birthday.
Not everyone has a life path that runs smoothly, especially if they've got used to taking up less space for a bullying partner.
You mentioned the weight training, that is FAR more important than you realise.
1 - you've got a rhythm to your life
2 - you've started weight training, which makes you feel confident, strong, and less inclined to be submissive
3 - you've finally found a course that looks like bearing fruit.
You've also had a child.
60% of domestic abuse begins during pregnancy or thereafter.
I agree that you've inadvertently walked into the web of a particularly unpleasant man, and need to keep your cool. Now he's let his cover slip, he'll be angry much more often, and it usually gets incrementally more abusive, whether louder, scarier, physical.
Get a plan, find all those ducks, but ffs don't start listening to anyone about giving up your course (or your weightlifting).
He's right, you have changed since the baby arrived. You've found your purpose, and the scales are falling from your eyes.
Be careful, keep posting wherever is best, and let several people know. Close friends, GP, parents. Anyone who can support you, and witness the timeline of his behaviour if it escalates.
Oh, and make sure you don't have mn anywhere easy to access.
I know I sound paranoid, it's just based on facts, statistics, and personal experience. My partner first "put me in my place" after about 8 weeks. He first got angry after 18 months. He first got me after 3 years. I very nearly didn't get out at 4 years. By then I had no idea who I was or what was wrong. It's stealthy, and very difficult to believe it won't get better if you just try hard enough (and do everything he says).
The GP, psychologist, and eventually psychiatrist all said it was textbook.
I truly hope that the combination of him showing his hand, and you starting to feel more confident, will mean you can bypass the months of problems most women go on to experience after this stage.
We're here if you need further advice or support.