Depends what he is mixed with , if a mastiff or similar type breed then yes large breed would be suitable. The APBT
isnt a large breed in itself but Like I said if mixed with one then yes you can. If he is growing to fast you may want to just take him off puppy food and put on adult food or all life stage food . We had a dog grow to fast and the vet had us do that, worked fine for us and he is perfectly fine now at 3 years old. 40lbs at 4 months the best guess is he will be 80-90 lbs full grown. At that weight I dont really think large breed most mastiffs and such get well over 100lbs. I would just put him on a normal adult food or all life stage food, quit the puppy kibble all together.