I have been through the process and, whilst my experience is not directly relevant, I thought it may offer you a little comfort or confidence!
My ex took me through the whole shooting match, mediation, Caffcass, back and forward to court, Social Services - you name it he tried it. The thing is, he was clever, he had never been violent and he knew all the right things to say. Cafcass saw through him, almost from the start. Not only that but the superior manager who reviewed the case saw through him, just from the notes! They were really sensible and really listened to what was being said, even when he tricked my daughter into saying things they got to the bottom of it with her (this was the point at which they gave up gving him chances - oh and the times he got caught lying to the judge didn't help much! ).
IME sometimes it does not seem as if they are listening to you but it does seem that the things you say get taken into account somewhere along the way - just not always where you expect them!
The only tip I would give you is something I have been told during subsequent contact with them, things that have been done against you are not always relevant to the way they see a person acting with a child. For example, someone being violent to a partner does not mean that person would be violent to a child IYSWIM. The best way to present these things is to highlight incidents when the child was involved or present.
I do agree that you need to let them know about his mother, they may well find that she has changed or whatever, but it is relevant to the proposal and it will not look to good if it appears that they have witheld important information such as that - it shows they are not being upfront!
I hope it goes well for you, it is a tough process emotionally as well as practically, just remember we are all here to help where we can