When I was reading Green Boy I thought of Girls of the Wild's for some reason. Sure, both of those series have strong girls and a boy who starts weak but gets gradually stronger, but truth to be told, I would count this recommendation as one of those "gut feel" ones. So, if you liked one of those two manhwa, you ought to check the other one as well.
P.S. As a bonus both of those series feature good and colored art, since both of them are korean webcomics.
Shirihama Kenichi is a weak scaredy-cat, and his status as the karate club's punching bag has earned him the name of ‘Weak Legs'. One day Kenichi meets Miu and, impressed by her skill after she effortlessly throws him to the ground, he follows her to the Ryouzanpaku dojo. There he finds five great masters of martial arts, and driven by his desire to become stronger, he begs them to accept him as their student - though it won't be easy. Kenichi endures harsh training (and injury) from Akisame - the philosophizing Jujitsu master; Apachai - the Muay Thai ‘God of Death'; Kensei - the ecchi Chinese Kempo Master; and Shigure - the weapons prodigy. Finally after proving his determination, he is accepted by Shio - the 100-dan Karate Master, who had initially wanted nothing to do with him. Now Kenichi walks the long and arduous path to becoming a master himself, all whilst trying to protect those he cares about from anyone who would dare to harm them.
Both History's Strongest Disciple Kenichi and Green Boy are about a physically weak boy who gradually becomes stronger. In both of those series the male lead joins a dojo (in case of Green Boy a boxing club). It should be noted that the fact that a girl he likes is a member of said dojo is a big part of the reason for joining (in case of Green Boy it's the main reason for doing so). Green Boy is more on the serious side while History's Strongest Disciple Kenichi has less romance and some fanservice, nonetheless if you liked one, odds are you will like the other one as well.
Both Kurogane and Green Boy feature a weak boy who joins a martial arts related club (kedno club in Kurogane and boxing club in Green Boy). In both of those series the protagonist actually has a talent for that martial art. Although Kurogane features significantly less romance than Green Boy, those two series do feel alike. You should definitely give one of those a try if you liked the other one.