GT bmx: change crank arm

I bought a used 2000 GT Box Frame Pro 3.0 and it comes with 180mm crank arms. Way to long for my son. I think he needs 155mm. So.. can I just buy the arms? Do they have to be GT, Any specifications I should look for. And Im looking on the less expensive side of this.
Novice BMX dad

You can buy just the crank arms but then you need to be sure that the spindle is compatible with what you have. I am going to direct you to http://www.vintagebmx.com/ Click on community and under the BMX Today section you will find the BMX racing forum. You will find an never ending wealth of info there. Also in the Vintage section you will find the pro's from when BMX roots grew. The likes of Stu Tomsen, Perry "PK" Kramer, Tim Judge and many more. Good luck. I'm 38 and I still race here and there.