Well, I think that it will be repetition that will eventually teach him. Definitely give the older kids their own space to do things like Lego, etc.

I do not think it is too soon to teach that "only food goes in our mouth". I think that as soon as they get to daycare, this needs to be implemented. Now that doesn't mean we get "angry" at the 12m old who just started and still chews on EVERYTHING. Just gentle reminders and removing the toy from the mouth to be wiped off. Some kids are more tactile this way than others, so if he/she is really struggling, I have been using the suggestion others made, of having a certain toy that the little one can chew on to replace whatever they are currently gumming. I have been doing this with my own 18m old who will NOT stop mouthing stuff, and it seems to be working!!