First of all, your son is A.D.O.R.A.B.L.
E. and he's even cuter because he KNOWS he's not supposed to be in there!
I went to Big Lots and bought a Rubbermaid storage container that is big enough to hold about 40 lbs of food. The lid snaps down to keep wee ones out, as well as the ultra nosy dog - almost too well because I have a hard time getting it off sometimes.
I've wondered if I should get something that is airtight but I just can't justify the expense. We go through the food fast enough that I don't think we have a problem with staleness.