Hi Rabid,
I guess it depends on his overall condition, genetics, his height etc. By the number alone, I would not say that he is underweight. Again, hard to tell without knowing more about the other variables. Do you have a picture you would want to post (if you feel comfortable with that). A lot of users on here have amazing experience, and can provide more detailed information.
My English Mastiff is a little short of 2 1/2 years and about 225 lbs, which was somewhat expected based on the parents. Even at three, Heracles can and probably will still gain some weight.