IMHO you need to either work TREMENDOUSLY HARD right now to stop this from continuing or you need to have him/her put to sleep.
He/she does look like a "pit bull type dog" (not saying it IS an APBT) and therefore if you do not do something about it NOW you will be adding another casualty to BSL.
I have been in a situation where an APBT was biting, growling and lunging at me for NO reason.....I persevered for too long......ultimately she was put to sleep (RIP Beautiful girl) before she hurt anyone more.
If you keep this dog (not saying you shouldnt but it may be a lot of work) and he/she bites or attacks a child or adult you are only going to be adding to the thousands of dogs who are identified as an APBT and PTS due to aggression.
FYI human aggression in an APBT IS NOT NORMAL!!! It is stated in the AKC (American Kennel Club) Breed Standard that any American Pit Bull Terrier who shows any human aggressive tenancies should be desexed and managed (obviously with a lot of thought being put into the quality of life for the dog) or should be put to sleep immediately.
I feel for you mate I really do! Big hugs from me and my dog x