First thing that you need to do is get an STI check. That should bring the circumstances into sharp focus. He made that string of choices- looking at another woman in a sexual way, talking to her, flirting with her, touching her, and going all the way. He repeatedly chose to not stop himself, through each step of that process.
I’ve been on MN for years and years and the going philosophy, imho, is that if you take him back now, that is pretty much admitting that you accept this behavior. He will do it again, sooner or later, but eventually yes.
His remorse and regret are cheap lip service to avoid having to start again with someone else.
I know you didn’t ask about the following but since you are new here: some more stuff for you to think about.
You have been with him 8 years and are unmarried. You have a child with him and are still unmarried. How is your financial situation? Please say you have your own stream of income into your own account and you are contributing to your own pension plan. Are you on the house deed?
At this point, I would say marriage would be the only thing that would fix this as it would show his commitment, as well as give you the financial security that you and your child deserve- if you are unemployed stay at home parent. (And be clear, marriage would not sway a cheater one bit. It would just put you in a better position financially if/when you do split up for good.)
And I will say it before someone else does: if you would enter into a marriage thinking the above- then best not get married in the first place as divorces are expensive and difficult. Unless you have contributed substantially to his wealth, perhaps just cut your losses and end this relationship now, that is all the sooner you can find someone you can trust.
Sorry to hear you lost your brother.