New Boss stereo

I just installed a Boss 508UAB stereo in my sons G5, install went smooth by the necessary wiring harness to keep the chime but I have run into an issue I can't seem to solve. Everytime the car is started the stereo is full volume, firstly scares the crap out of the driver and I can't figure out why it is doing this. Any one run into this issue? I know the stereo isn't the best but its what my son can afford currently. Any help would be greatly appreciated, I'm worried it will blow the speakers if I don't get it resolved. TIA