There is no way for anyone to tell you which company will be cheaper for your son. Anyone who says, "This company is cheapest," or "That company has the best rates for young drivers," has zero idea what they are talking about. There are far too many underwriting variables involved for one person's price to be relevant to another person's price. The only way to accurately compare prices is for your son to get quotes for the exact same coverages from multiple companies. Please keep that in mind when you read some of the inevitable replies to your thread.
As far as pros/cons of various carriers, you can find people who have had great experiences with all of the companies you mentioned, and you can find people who have had poor experiences. For example, I work for Progressive, have been there since 1988. I know that in my 20 years in the claims field (1994-2014), I provided people with fantastic service, and made their lives a little easier after they'd been in an accident. I also have encountered situations where we did not live up to the level of service that a customer should expect. My wife was hit by a Progressive customer a few years ago, and they took great care of us. In my current role, my team works with every single major carrier, every day. I can tell you that I wouldn't have Geico insurance if it were free. I think their claim service overall is poor, and the vast majority of claims people I have encountered there are rude and unprofessional. Others may have had good experiences, but mine is based upon thousands of interactions with them. I also would never insure with Allstate, but that's because of their long history of unethical practices involving homeowners insurance, not anything to do with car insurance. In fact, one of my wife's co-workers had Allstate, and dinged my wife's car, and the claim service was very good. Some of the state Farm Bureaus are good, others (IN and TN), I would avoid like the plague.
State Farm, USAA, Liberty Mutual, Farmers...all provide good service, though State Farm is really struggling on the auto insurance side when it comes to pricing people accurately. Hartford is a good company, as is Cincinnati. Erie and Westfield tend to be a little more selective when it comes to underwriting, but they are both good companies who provide good service. Mercury is a solid mid-sized company. I've had mixed reviews about AAA. If you would like to know my experience with any other carrier, fire away. I've worked with just about all of them.
You can buy direct, or you can buy through an agent. I have an agent only because that was the only option to purchase from my current carrier (MetLife). When I compared quotes, they were so much cheaper than everyone else for the same coverage that even though I prefer to buy direct, I couldn't pass it up. Their claims service has been terrible, and I wouldn't recommend them for that reason. I've had no issues getting them to do what they were supposed to do, but I am certain that if I didn't have a lifetime of experience in the field, that would not have happened. When I switch carriers, I will likely buy direct.
