Lawn with sink holes

[kalyeah]

Hey guys, I just picked up a customer(lawn mowing) that has a couple sink holes on his property. I don't think the house is more than 5 years old. He says he's filled them before and they just sunk again. Also looks like the ground is sinking some around his driveway. There's a gap under his driveway. Should I just keep filling the sink holes and tamping down good? Should I add some gravel for drainage? Maybe he should call the builder concerning the driveway? Thanks in advance.