No different to peting a deer. They can be tamer, like deer at a petting zoo park. Most people don't tame the boomers: If he is a kid, it is likely a joey (juvenile) or a wallaby(smaller species) and likely at an animal park petting zoo. If the roo is taller than a grown man then rare but not impossible tamed red kangaroo boomer and not during rut. Deer can be just as dangerous.

Incidentally, roos taste similar to venison too. Good healthy lean meat, better for the environment here than hoofed animals, which cut up the ground and damage the native plants.