If other travel and PAL teams are using long poles in 4th grade then you would be a foolish coach to discourage your player from using one. With that being said, there is absolutely no reason to get one a year early to get used to it. He'll get used to it in 4th grade just like everyone else.
"Because other teams are using long poles" is a terrible reason to allow your 3rd/4th graders to use a pole. Let the kid develop his feet and instincts instead of relying on the pole as most end up doing. Ignore other teams and let the kid learn to play D with a short stick. Just my 2 cents.