Our 4 year old dog is protective of our new baby grandson, do you have any suggestions?

We have a wonderful, caring 4 year old male Belgian Sheepdog. I've done obedience training with him professional trainers over time for general commands (heel, with me, etc.). He can be oversensitive to stimulus. He is great with adults and now is dealing with his first exposure to a young infant (our grandson). He is becoming quite protective as he darts at our other dog if he goes near the baby. He even darted toward me while I was shaking a rattle in front of baby's face. He was going directly toward the rattle but didn't touch me. I having been giving him a firm NO when he does this and sometime insist on down position. He seems to be overprotective. He loves to lick the baby and nudge him to sniff and check him out. We never allow our Belgian near baby without supervision and after a couple of licks and greeting we tell him to leave it. Any suggestions? I've been raised with GSD, so familiar with the instinctive protectiveness. Think our Belgian is over-reacting. Hope our approach will not make this more so. We do say good down and good boy when he listens and "leaves it" (retreat). Thank you for any suggestions.

Joanne