Welcome to HCM,
It is not clear from your question that, if you have used condom, why are you worried about fertility
of diabetic. Whether he is fertile or not depends upon his Semen examination report. Your pregnancy
seems to be due to sex with your partner only. In a regular cycle of 28 days, ovulation occurs on the 14 day. You calculate 14th day of your last menstrual period. With whom you had sex in the vicinity of 14th day, can give rough idea about person who might have made you pregnant. If it is with diabetic, your child will be more prone to suffer with the same disease.
Wish you good health,