A70 kg boy is sitting 3 m from the ground in a tree. what is his gravitational potential energy

Potential energy (Ep) is equal to the mass (m) of the subject multiplied by the height (h) from sea level that its positioned multiplied by the earth acceleration (g).

Ep = mgh 
In your case m = 70 kg, g = 9.8m/s^2 , h = 3m , all the units are in SI, so simple multiplication gives Ep = 2060 J or Nm^2