Apples & Bananas, I think what you are doing is right to try to get him to stop but I think I'm more like Littlefeet and don't usually worry about trying to get them to stop mouthing toys until they are a little older I guess. It does get old having to remove all the drool toys for constant cleaning, I hate it too.
I have had some very 'oral' children in care and I definitely work on them as you are. I have a boy who came here at 2.5 years old in the summer who I had to wean off a soother for pete sake and then he started sucking on his entire hands and I've almost cured him of this too. Ugh, I hate it!