There is a huge difference between physical therapy, and mental therapy. And perhaps Ben was never so unstable as to cause any concern. Heck, there are people who are quirky, who are considered eccentric, unconventional but that doesn't mean they need a psychiatrist.
In Ben's case, his troubles have been piling up ever since his father came to Salem, and then his sister left town. Daddy dearest was a fraud, pleaded being sorry for what he had done to the kids in their youth, convinced Ben (tho not Jordan) that he was a changed man who wanted to make it up to them all. Ben took back the family name, believed all his old man said, had no idea the evil the man spread throughout the town, or what was said and done behind his back, in the name of "love" for his son.
So.......his sister left town, his father filled his head with lies, he fell in love with a gal who actually was in love with someone else, but would not admit it. He became more and more paranoid, Daddy dearest was hauled off to jail in Florida, and Ben was left to flounder around on his own.
And, he just edged closer and closer, finally falling off the deep end.
I think Abby realizes he is not stable, that his thinking is very marred, which is why she keeps referring to the baby, as he seems to want to protect the child, but in his own way. Which truly is not the right way, but he thinks it is.