Some people poo-poo the idea, but I've heard "runts" are sometimes just conceived later in the gestation... so they're just a little behind on the times.
Size is one of those genetically programmed things - unless, as DMikeM mentions, there's another medical issue going on - so even a puppy with a slow start can catch up and surpass expectations.
Given good food in good quantities (not too much... over-feeding still just goes to fat, which is unhealthy for bones and joints) the puppy should catch up. The slower the growth, the better, so don't worry about it, and keep the puppy healthy and lean, and he'll get there when he gets there.
For these giant breeds, be careful not to over-do calcium, too... puppy formulas like to add extra calcium for "growth", but puppies under 6 months can't self-regulate blood calcium levels, which means extra can plate out on bones and joints, causing long-term issues. You want a food with <1.5% calcium.
Do you have any pictures? We like puppy pictures around here!
