Prey drive in English Mastiff?

He needs to meet a few smaller dogs. I'd put him on a prong collar and correct him for showing too much interest in the dog (he's a big dog, he requires his size and energy be accounted for). If you give him a correction under his energy level it'll do exactly nothing to switch his drive off. Once he's calm you can move the dog closer to him paying close attention to body language and his energy level. If he keeps acting silly, slow every thing down and be sure to give the corrections on time, before he gets into "Must kill!" mode. Doing this a few times will teach him the energy he must bring when meeting a dog, big or small, and will teach him how you expect him to act. Of course you may muzzle the dog if you're not comfortable having a smaller dog that close to him and if you can't manage his size you might actually have to put a harness and leash on him plus the training collar (and separate leash) so somebody can help you hold him via harness while you correct him via training collar. I actually had to do that with a mive APBT x Mastiff mix...Pitbull strength + Mastiff size = bad news! Anyway, good luck with your giant puppy! I hope I helped feel free to e-mail me with any questions or concerns or if any new problems pop up in his training!