I think he needs to know of the possibility. I agree that you should go to your doctor and let your mom know, these will be supports for you.
For your ex, you could just say to him something like "i don't want to alarm you, but i've missed a period and i'm concerned." tell him that he doesn't have to stay, that you want the best for him, ask him for a phone number and address. Tell him you'll keep in touch. Be honest! Tell him that you've thought long and hard about telling him. Tell him that you don't want this to be a burden on him, that you only thought he should know. Let it go from there, and don't worry too much about it.
If you'd rather not go that route, get a test real quick before you tell him. The test takes 5 minutes. Even if you go in to your doctor, unless they do a blood test, the results will come very quick.
The plus to going to the doctor is that he/she can tell you the margin of error. Provided you give accurate dates, they can tell you right away whether this is a false reading if it comes back negative (positive results are almost never wrong).
That way if it comes back negative, you don't have to tell him a thing. It's your call, hope all goes well.