My partner had a horrific childhood. It has given him a deep understanding of the damage horrific parenting can do and has made him determined to give our children a happy childhood. But it has also made him very insecure and angry, and without a model of loving family relationships.
So when he feels bad, he rages at the children for minor stuff. When he wants something, he is unable to ask for it but has to manipulate to get it. He is so afraid of making mistakes that he lies about the stupidest things. He asks me and the kids constantly if we love him, puts me through cruel emotional tests to see if my love is real and can get jealous of any positive relationship me or the kids have with others (even with each other, sometimes...). I.e. he is a typical emotional abuser. Many of those had, I'm sure, a horrific childhood.
I am still with him because he tries to get better. He is on medication and goes to counselling. He responds well to me setting boundaries. We are starting family therapy soon. But man... it is exhausting and cannot go on like this forever.
Me and others telling you to be careful are not saying he is a bad person. Just that horrific childhoods create horrific damage that does not go away with just love and support. It requires hard, hard work from him and even then it can be a long road, that is not necessarily successful.
Anyway, as for support: you can support him by modelling healthy family behaviour and not letting behaviour you wouldn't accept from others slide because you feel bad for him. On the contrary, pull him up on it and let him see what healthy people do when they are angry, hurt, sad or frustrated. Of course you can also listen to him and let him cry when he needs it. But really, that alone will not be enough to make him better.