I agree & wouldn't even mention the dog as a concern. That is nothing compared to the safety of your child. That is what I'd be stressing when you talk to them. You moved into a house with a secure yard for your DS's safety & it is no longer safe for him.

Can you look more at houses out of town? If you've got a chance of going more to the outter area's, LL are less worried about dogs, because there's generally more room to entertain them, so less digging/barking & anything else that can be a nuisance.