How big will my bulldog/mastiff puppy get if he weighs 28 lbs at 18 weeks old?

My puppy has gained 11 lbs in the 7 weeks that I have had him.. Currently he is 28.2 lbs at 18 weeks old and we are unsure of his breed. We know he is definitely part french/english bulldog and the vet believes he is mixed with mastiff because of the size of his feet and his current weight. Any suggestions as to how big he will get when fully grown?