Well, you don't want to dilute his "go to bed" command, so I would say actually have a bed if this is what you want to try. I think you could try the kong, could be a good thing.
At this point, I wouldn't randomize the cookie tossing. I think whoever is holding him should toss the treat--the less focus on you the better. But I wouldn't randomize at this point in time--I would have the person holding him throw on every time, until he starts looking for the cookie as soon as you leave. I wouldn't make it obvious either, just a suddle, "Whoops! Treats just rained from the sky, look at that...." That way he thinks every time you leave, treats fall. If you repeat this enough, he will start looking for treats when you leave. That would be the point where you start to randomize it a little bit. Maybe toss one 3 times in a row when you leave, but then not the next time. Then maybe 2 times, but not the next two times. Then maybe 6 times....completely random, but when first starting out don't have a big gap of treats 5 times, no treats 5 times. Just start out with one or two no-treat times, and follow with times where he does get treats.
I definitely think varying the location could help too, so he doesn't think, "Mom is ONLY going to leave me by the classroom." He will be more solid and will get more out of it if you vary the location. Just remember that in a different location, you may need to take several steps back and make it a little easier on him. Maybe talk to the people at day camp and use this exercise there too. Have whoever takes him drop a treat as you leave so you are further reinforcing the calmness/quietness when you go away.
Is it the distance that bothers him, or just watching you leave? If it is watching you leave, then I would say you don't necessarily need to focus on distance. Certainly wouldn't hurt to work on it, but if his only issue is watching you leave, then I would focus more on that.