What Can Cause Bruises On Body?

my brother is 16, overweight, and diabetes runs in the family, today he showed me 3 huge love bite looking bruises, two on his upper right arm and one on the upper left chest, strange looking deep purple and red bruises which are oddly shaped, could this be either a tissue damage from a while ago just surfacing or is it medical related like cushings sydrome or similar, im a little concerned that its an early sign of something? or serious harm by friends as it may also be a case of.