agressive behavior

Question
My 7 month old Sheltie plays nicely for awhile then all of a sudden get very vicious. I need to know what i can do about this because he has growled at my son.  I can't let this happen and really don't want to get rid of him.

Answer
Actually, consider yourself lucky that your dog DOES growl at your son.  Dogs that are reprimanded for growling at children sometimes stop growling, but then their only defense if they feel threatened is to bite.  The way you protect your child is to protect your dog, so do supervise all their interactions, and never leave a child alone with any dog, even the most calm dog.
A 7 month old pup is an adolescent - think teenager.  If you haven't already done so, this is a good time to take him to a class for a refresher on basic manners.  If you don't have time or can't afford class right now, there are some free lessons at www.clickerlessons.com.  Shelties are really good at clicker training, and it's hands off training, so safer for kids to participate in - if your son is old enough, he could try some of the exercises, too, which will help the dog respect him more without any conflict.
There's a great book, by Coleen Pelar, called "Living With Kids and Dogs Without Losing Your Mind" that I urge you to get.
Unless your dog is suffering the kind of sudden viciousness associated with neurological problems, I suspect that what you are seeing is just behavior related to his testing limits.  Training probably can help, as it will give you a vocabulary to use with the dog - as an example, my Aussie understands "leave it" to mean "don't put your mouth on that", and I can now get her to "leave" a tennis ball, not bite her sister's ear, not touch my toast on the hassock, not try to herd any kids, etc.
(The lesson site I gave you has a lesson on leave it.)