I've only had one dog at a time but when mine started I bought him a big teddy bear and encouraged him on it. He's never humped anything else. I wonder if that would help? It's hard to just make them stop doing that kind of behaviour so directing him to something that's allowed might work but I'm guessing. I know it works when there is only one dog in the house but I'm guessing when there are two and the older allows it. Whether you redirect or not, I'd stop it. He'll try that with some other dog and get told off, maybe badly.