If you're noticing the "look in his eye" when he's chasing a dog. Stop it. Stop the chasing right there. Get him on leash, give a correction, wait for him to calm down, and then let him go again. If he gets back to chasing, repeat.
I do not let my boy chase other dogs for anything more than a minute. It tends to not end well because at some point either the dog gets freaked out or their owner does. The way your dog is currently acting, will not end well for your dog. This is a HUGE problem. To put it lightly...if your dog acted this way towards my dog, you wouldn't have time to pull him off or tell him to get off before you'll be trying to pull my dog off of your dog.
This isn't sexual humping, its dominance. And since you're good enough to notice the "look in his eye" you'll easily be able to tell when he reacts to a female in heat. More than likely, you haven't seen it happen as at that point its highly likely that nothing will get through his skull.
Again, stop allowing him to chase dogs for that long, and do not ever allow him to hump other dogs. You should always be close enough, or have enough verbal control over your dog to prevent him from dominating other animals.