The only time with your boyfriend was May 1-3 and the only time with Mr. Wrong was May 16? (You're saying you didn't have sex with your regular boyfriend after your period came on May 5 and before you found out you were pregnant?)
Have you had any ultrasounds at the doctor yet? If so, what due date did they give you?
It sounds from what you have said that if you didn't have sex with your boyfriend after your period, the guy from the 16th is the father. This would be less certain if you kept having sex with your boyfriend throughout the time period.