If they are a TRUE subcontractor, then they need to get their OWN insurance - general liability, and workers comp.
If you tell them when to go, where to go, and how to do the job, AND you pay most of their income - they aren't a sub - they are an employee. Calling them a sub doesn't MAKE them a sub, neither does failing to withhold employment taxes.
Also, if they don't have workers comp, and they get hurt, either in the car going to or from a job site, or at the persons house - YOU the GENERAL CONTRACTOR are legally responsible for their workers comp coverage - that would be, medical payments, rehab costs, and lost wages.
THEIR insurance isn't going to cover YOU, especially if you call them a sub. So you will ALSO need your own general liability coverage.
You need to contact a local, independent agent near you to get insurance quotes. As a startup company, you are NOT going to have your pick of standard companies - State Farm, Nationwide, Zurich, Allstate, Travelers, ALL want three years prior insurance coverage in order to take you on, so you'll probably end up in the excess and surplus market - so if you REALLY have this guy as a sub, expect EACH of your gl policies to cost about $1500 a year, EXCLUDING PROFESSIONAL LIABILITY.
And do check out the irs website, to see if you can honestly pass the "subcontractor" test. http://www.irs.gov/businesses/small/article/0,,id=99921,00.html