Son left e300 out in rainstorm and wont start, no on light, totally dead. will not charge, will not do a thing. what is the problem! It had been on covered back porch all winter, and he would use it once in a while and it did work a few days prior to being left in rain. Just bought last summer.

You don't say what brand e300, buy the connections are corroded. Doesn't matter how good it was taken care of previously. Being left out in the rain is all it took. This is why electronic devices TELL YOU to not expose them to moisture...
No telling which circuit board is fried, but if you solder, you can replace them one by one to find out.
But TBBH, for the time, aggravation, and cost, just go buy a new one if the original cost anything under $100.
If you paid more than $100 for it, call your local RC shop and ask who does repairs.
Could be your rotors (most likely - copper rusts by breathing on it), or a board connection. Rotors are not hard to replace if you can solder.
JMO, but if it is of value, always supervise kids when you let them "PLAY" with it...But hopefully, lesson learned all around. Best of success to you.