Welcome to YT.
Hard to say without seeing your dog and hearing the grunting.
Do you use a harness and not a collar? Does he pull on the leash or walk easily?
Is he the correct weight for his bone structure? If he is overweight, the grunting could be from exertion.
Grunting could be related to anything from excitement to leash/harness pulling, or worst case, extra weight or collapsed trachea.
How long has he been grunting?