I had the same exact problem with my 7 month old Boris and he had been humping since 8 weeks of age. He would hump everything and even tackle children and mount them. I've been working with it for about 2 months now and he's made a huge improvement when around other dogs and children.
When Boris humps I will push him off and say no. If he gives me any attitude about it I pin him on his back either with my hands or with my mouth (Don't recommended mouth for everyone) around his neck fat until he submits and calms down.
I also bring Boris to my parents house regularly to be around other larger dogs. If he tries to mount them they will surely snap at him which has also helped him realize he can't just mount anything he pleases.