I can't really see what the problem is with saying, "No, that's a bit too salty for him."
If your inlaws say, "Oh come on, it won't harm him," you can just say, "Actually, it's not recommended, so I'll just stick with the guidelines." If they say, "Ooooh, mummy's so mean," just smile and say, "Yes, terribly, terribly mean."
You can make whatever decisions you feel most comfortable with for your child, but it seems a bit much to expect to control or avoid any commentary on them as well. If you feel it's the right thing to do, I don't really see why you'd have a problem sticking up for your decision.