The dog over the fence got into my yard and attacked my dog?

next doors dog got into our yard. Both dogs are similar in size, mine is a pit bull, mastiff x theirs looks to be something similar. It some how got into our yard and attacked our dog (who was chained up because he was sick with parvo and he digs out sometimes and we did not want him to get it again) A guy (that was not the owner ) jumped over after the dog and was trying to pull them apart. I was inside bathing my 1 yr old when i heard the noise, i went to the window and seen the 2 dogs going for it and the man trying to pull them apart. He started yelling at me to help and grab my dogs chain and pull him away, but i had to get my son out of the bath first. any way our dog had some cuts on his head above his eye that got infected and he had to go on a course of antibiotics. the antibiotics ended up costing $66. The money is not a big deal though our cat has gone missing and i now have not seen him in over a week. And i think i heard one of them say something about the cat to someone else over the fence oneday when i was hanging out some washing. but i cant be sure. another thing is some times when the back door is open my 1yr old likes to go out side and wander about (which should not be a problem the yard is baby proof high fences and a gate that is locked almost all the time) i am terrified that this dog will get into the yard and if it got my son im more than sure he could kill him i should not have to worry about this happening its my backyard where children are ment to play! is there any thing i can do ?