This may be a bit on the small side for 1 yr, but I wouldn't stress out. He still has a LOT of time to grow. DDB's really don't quit filling out until around 3yrs most of the time.
I have a 6.5 month DDB and worry about the same thing all the time (because I want a BIG dogue, otherwise why get a mastiff?). He is 73 lbs and I've read so many people online who have "100 lbs at 8 months) or "90 lbs at 7 months" and I wonder if Louie is going to be small. I think he'll be fine, and the bottom line is he is a beautiful, healthy dog.
Remember also, DDB's should be ATHLETES FIRST, large second. I'd rather have a 135 lb athletic dogue that can run, jump, and such, than a fat 165 lb dogue that just lays around but I can say "Hey I've got a 165 lb DDB!". Think about it.
Hope this helps.