This assumes a 'no fault' break up, ie: you've realised you aren't right for each other, but no one has done anything egregiously terrible, liked cheating:
Do it face to face (unless you have reason to fear violence)
Be calm and clear -- "I've been thinking about it a lot, and I don't think this relationship is making either of us happy. So I've decided we should separate."
Don't give mixed messages or ask for a break/some time apart to think/a temporary separation -- it's kinder to be clear rather than give false hope.
Answer any questions honestly -- they may ask if it is something they have done, if there is somebody else, if you are sure this is what you want, etc. but try to avoid blaming them or getting into a slanging match.
Remember that you don't need to get them to 'agree' with the breakup, or your reasons for doing it. Don't waste time trying over and over to get them to see your point of view -- the takehome message is 'this relationship is over'.
Give your partner and son time to find accommodation, and offer to help, but set a firm time limit and stick to it.
Set up some RL support for yourself.Even if you are the one who ends it, breaking up is still sad and difficult, and you will need some compassion and care from your friends.