There is a chance you can still get STD's with a condom. Nothing is fool proof except abstinence. Think about it... The condom may protect himself from you but are YOU really protected? Also remember condoms break.
I'm going to have sex with a guy who's had sex twice. I can't tell mum anything and that limits my options for contraceptives or any STI check-up. If we have sex with just a condom will it prevent the spread of STI's? Even if he's had sex twice?
He said only twice ? Most people downplay the amount of sexual partners that they have had. Don't let that sway you into a false sense of security.
No guarantee that a condom will prevent diseases or herpes.
Your mum is your best source of help. Pity, you are throwing that away. By all means, go to strangers on the Internet. Brilliant.
3rd time is the charm---what do you think might happen?