You could call the vets for advice but I don't think I'd take them in unless the vet advised it as they are still so young. It may well just be that ones the runt and a bit slow developing. If mum will let you without getting upset, you could hold him to her to get an extra drink to make sure he's getting enough milk. Just have her sat upright (distract with food) and hold his nose near a teat.
It might also be an idea to weigh him, that way you can be sure he's growing still even if he's a stage behind. Does he have normal proportions (head to body ratio etc.)?
Unless he's showing signs of illness i.e. discharge, bad droppings, an injury, not feeding, too hot or cold etc. I don't think there is much a vet can do.