Don't help Bobo (the homeless guy at the start) and you will have a chance with a relationship with William instead.
There's variations on some scenes depending on whether you are submissive or dominant as well (like with Mark).
I have several different saves for different routes.
I believe that Myriam can still get with William later on, even if she helps Bobo. If I'm not mistaken, helping Bobo just cuts off the earlier content with William. After his mother goes into the hospital, I think there's a second chance.
I'd have to look at the game code to make sure but every playthrough where I help Bobo and miss that initial William scene, all of his content is inaccessible forever more.
You can help Bobo, you just need to be fast about it, JUST go for clothes and you will see William in front of the house, but if you go for clothes AND food, you'll take too long and miss him.It's a bit of both: You can still get some content with William, but it will be significantly reduced.
And you don't have to skip helping Bobo entirely, just don't be too long helping him. It's finding some food for Bobo that blocks off most of William's content, if memory serves.