He is not your friend. But there is no harm pretending and acting that he is (for now). Keep everything close to your chest - go see a solicitor ASAP but do not tell him.
As others have said - screenshot and photocopy absolutely everything you can get your hands on for his business and your personal finances…….savings, pension, investments, assets, etc. it’s highly likely he will try to downplay how much you have and how much he earns when it comes to a settlement.
Do you get child benefit? If not apply for it online today, as it can take many weeks to process. Also start a child maintenance claim immediately now that you are no longer together. See what benefits/UC you will be entitled to and again apply immediately.
Start updating your CV and looking online at potential jobs to apply for. Speak to your solicitor and ask whether it is in yours/child’s best interests for you to seek a job asap or to wait until after finances/divorce are sorted.
I know everyone jumps to an OW, it’s not always the case, but keep an open mind - the fact that there has been no discussion from him previously to say he was unhappy, attempts to make things better, suggest counselling, points towards an OW. The timing of a Boxing Day break up suggests an ultimatum and him saying “I’ll stay just to give my child one last Xmas.” Seems unusual timing if there isn’t an OW.