Not sure there is much I can add.
Yes you have both been irresponsible, not that, that helps now but probably good to acknowledge that for the future.
If you do not want an abortion don't feel pushed into it. If you decide to have the babe then the choices are as you lay out, convince your partner this is a permanent relationship to get him to accept his responsibility and work to make this... well work. Or to accept he isn't going to and accept your role as a single mother for a second time as a consequence of past action or inaction.
As for telling him, you just have to say you two need to talk and... well just tell him as you have told us. You can't avoid that, you just have to take a deep breath and do so.
Best of luck with the outcome.