I have a 15 months nephew. He seems so angry these days and very emotional (crying and shouting) what can we do? And is there any physilogical cause?

Tantrums? The only way to see if there's a medical cause is to see his doctor. He may be having tantrums. Often, kids act out to get attention, even negative attention;withdrawing attention from the misbehavior (i.e., timeouts) & giving positive attention for good behavior helps. Distracting your child when you see misbehavior start can head it off, too. Ensure he doesn't get overtired or famished.
Possibly. This is not normal for a 15mo. Something is clearly bothering him and a helping professional with experience with young children should be seen.