Most Helpful Guy
So if you've already met him in person and he wants to meet up again, why are you worried. It couldn't have been that long ago that you saw him and you couldn't have gained that much weight since then. If things develop more between the both of you then he's eventually going to see your arms. A good guy should be able to look past something like that. He's obviously into you if he wants to meet up again.
Most Helpful Girl
I don't think you have anything to worry about :)
He has already gone on a date with you, so he has a pretty good idea of what you look like. Stretch marks are not something that will scare a guy off unless he is very superficial.
Try not to let your insecurities affect how you think others feel about you. Most times other people do not even notice the things we hate the most about ourselves. The bottom line is, this guy sounds like he likes you. He's going on a second date with you, which is a really good sign! I wouldn't worry so much about your stretch marks, he will probably not even notice them :)