Hi K....
I study child psych and i, too, have a toddler. Sorry, this is a teensy bit long winded...
My little food thrower is now 24 months... it gets better. I believe it's rather important to get a hold of the situation now... before he gets bigger & makes yet even bigger, more intentional messes.
I taught my son some baby signs (by the way, i highly recommend!!)One important table manners sign is "finished". He still uses it today, even though he says it, too.
I believe it's important to sit and eat with your son – even if it’s just a light snack--especially during his early years. Just make time, if you don't already. Our children yearn for our undivided attention and meal time is a perfect opportunity to just get to know one another… take advantage of this break to relax for a few moments together with your young one. Your son learns from watching you & mimicking you and if you aren't there to show him how to sit and eat and keep his food on his tray, how do you expect him to do so?
With consistent, firm & gentle phrases i still (& have since my son was able to sit up and eat at his high chair) remind my son to: say "finished" when he's done rather than just stand up in his high chair or at the table, also, i remind him with, again, consistent, firm & gentle words to "keep your food in your mouth or on your plate/ tray" (which ever applies). Say 'please' and 'thank-you' when asking for 'more'... etc.
My son is a joy to take out to restaurants and family gathering because i know he will behave and he has displayed his table manners at home.
It is totally natural for children to play in their food... i encourage the exploration, but it is totally reasonable to expect them to keep the food on their trays!! You’ve got to show him how.
Positive direction works wonders! Tell your son what you expect and what TO DO rather than what not to do.
Your 18 month old is, actually, according to several recent psychological studies, a heck of a lot more intelligent than scientists had ever before imagined! He just needs a little help with the communication part along with the guided one on one example.
Good luck, You can do it!
D., proud mother of a terrific toddler!