You really don't want advice from me on this. This is one reason why it's better to wait until you are in a committed relationship before having kids. There are a lot of ups and downs, and mood swings aren't even close to what is ahead. If you are worried about him leaving you over mood swings, how will you feel when you are throwing up all of the time, When your belly is huge and covered in stretch marks? What about when he watches a baby come out of you?
And that is all of the easy stuff. Once the child is actually here, the stress really begins. Will he stay when a baby is crying at all hours of the night? Will he be changing dirty diapers with you, and wiping baby throw-up off of himself with a smile?
Will he stay when you two can barely afford to pay the rent, electric, heat, phone, car, and insurance bills? Food bills? Will he get that second job in order to pay for all of it?
This is real life. You know him better than I do. I hope it all works out.