A There's nothing to argue about, and you're not asking for permission. You're simply stating the facts, one parent to another: You're the one raising your son for the next two decades, and you have to be the voice of authority for him. Telling your father the why and not just the what may persuade him to back off. At the very least, it will cast you in a more adult, parental light.
When grandparents meddle, it's usually because they still think of their grown children as kids, rather than adults with children of their own. Even when they can make the mental leap, they have to know when to bite their tongue and stifle their advice, and when to jump in and help. This is a balancing act that can take years to perfect.
"For many new parents, having a child becomes the first time they have to discipline their own parents," says Wendy Mogel, Ph.D., author of The Blessing of a Skinned Knee. "You should always start out by being as respectful as you can, but you have to give up needing to please." It's more important now to behave like a good parent than a good girl.
At the same time, remember that your dad did raise you, and you're still standing. Tell him you value his experience and ask him to tell you what he wishes for his grandson's behavior, and that you'll communicate this to your son at the appropriate time. (The truth is there's not much you can do to discipline a toddler. Childproofing, distraction, and regular naps are the keys to good behavior right now.) You don't have to take your father's suggestions. Many grandparents tend to worry that their grandchildren are being raised too permissively, which is mostly a difference in generations, says Mogel.
If Grandpa continues to scold, pick up your son and carry him out of the room as soon as it begins. Ultimately, removing the audience may be the best way to teach your father to speak more wisely.