I would think that if you started the conversation with "Before we do this/sleep together(or however you word it depending on when you tell him), there's something I have to tell you...." He'll be relieved to hear you mention scars rather than an std.
Really though...
I don't believe I'd wait until THE moment to mention it, though. That could definitely dampen the mood because it would force him to focus on something other than sex. It would probably have nothing to do with seeing the scars, just throwing off the mental thing.
As for me, I kind of believe that telling a man fairly early in the relationship would serve two purposes. It would stop heartbreak later. If you found out he had a problem with it, it seems to me that knowing that information up front could avoid falling in love THEN finding out he couldn't handle a few scars.
The other purpose, for me, would be to weed out men I don't want. I, personally, wouldn't want a man who would have never considered dating me as a heavy person. That would be a deal-breaker, and I'd send him packing. I'm not interested in a man who has a prejudice about heavy people. To me, that is a character flaw.
Any man I would consider would be the sort of guy who would view my journey to health as a battle won and, thusly, a reminder that I am one strong, capable, and motivated woman.
I'm thinking telling him it was a gall bladder surgery would work for a very short term relationship. BUT, if the relationship grew, you'd have started it with a lie that will be exposed in all the old family pictures. If I were the guy in that situation, the scars would be a non-issue, but the lie would be the reason I would walk.
I've been a big girl all my life. I'm married to a man who married me as such, loves me, and thinks I'm the sexiest woman alive. BUT, before I married him, I dated a lot. I really believe having personality, taking care of yourself, and showing confidence are much more important to many men, and the others don't deserve the time of day from goddesses like us!