Ok, as for confidence, I think everyone else has covered that topic, and so you already know both of you have to work on his confidence. And I know that will take time.
One thing I have seen in a few different places, is the acceptable dress, and you keep saying jeans are just what people wear in your area. There's nothing wrong with blending, if it's working for you. Dressing to gain interest is not always dressing like everyone else. It's about standing out from the rest to enhance the projection of yourself image so that others are more interested. Sometimes when people dress like everyone else, they don't get noticed. It happens to me at work, as well as parties. If I don't dress a step above the people I supervise as a business professional, some people view me as a college kid even though I'm knocking on the door of 40 yrs old with 15 years quality experience and credentials that can stand against anyone (but if I don't step it up, I know I'm just perceived as a college kid). Sometimes when a person is struggling with some aspects of a negative self image, he/she may find himself/herself not obtaining positive interest of others. Then it's time for change so that you project a more appealing self image, you can't stand behind "that's what everyone wears." I find often that some of the people that stand out to us at theme parties, are often the ones that don't go with theme. With alot of those people not following the theme, I can't help but think, yeah they are sexy as hell, but also ask myself, If they were dressed like everyone else, would I notice them? or would they blend in? If it was me struggling with obtaining interest I wouldn't wear a suit, but I might try some dark colored casual dockers or kakis with a flashy bright colored shirt (or something different). I checked out the profile on SLS, and I hope he smiles more than I saw in the profile. If he starts getting noticed this will also help his confidence.
If normal in your area doesn't make him draw interest when he approaches women at the parties or people at any setting. Why would you be normal? I don't know about you as a woman, but as a guy, I don't want people thinking "he's ok", "he's a guy", or worse, "I'll put up with him, to get to his wife." Normal works for some people, but when it doesn't work for you or him or both of you as a couple, don't be normal. Get him to be different and get him noticed. Cause right now, normal sounds like it sucks for him. The other things will help as well (i.e. confidence, weight control, basically other things covered by other people). You actually mentioned somewhere that other guys his size didn't get much attention either. So if he's being like or dressing like them this just adds to my point. There is no one answer for anyone, for him it might not be dockers and a different shirt, but don't cling to jeans, just because that's what people wear. I could get away with wearing an SEC College Football Golf Shirt at socials here, but I'm probably just going to be another guy in Alabama. I can't stand out trying to dress like everyone else.
So, Step it Up! and Don't stand behind being Normal, when Normal doesn't work for you! Stand out or be weird in a positive way, that works! Cause it sounds like normal sucks right now. It's time for a few changes.
That's my 2 cents for what it's worth.