I have a girlfriend who has a sling like this: http://www.theslingset.com/ that she says is super comfortable. I have never heard of any problem with carrying boys in a sling, I cannot see the genitals being squished that way any more than they are when they are being carried on mom's hip. I carried both my boys on hip constantly with no complaints.

One thing for sure about boys: if the package isn't happy the boy isn't happy, so I'm quite sure your ds would alert you to any discomfort! I know the one time my ds's carseat strap got twisted & we didn't notice, it was way too tight on his crotch and he started fussing and complaining til we took him out to see what was wrong.