Behaving Very odd for 1 to 2 mts. That.

Hello Doctor, Actually my friend's son is facing with a different problem.. Could you please advice whether 1. its a problem or not 2.If it is a problem to which doctor we have to show him Problem Description: My friend has only one son..He is 6 yrs old..Financially well settled family..My friend is house wife and her husband is working in software side..They have only one kid..My friend has Polio on right leg . she can do all the things on her own only problem she can't run. She is a yoga teacher.Her husband is early diabetic. He got diabetes at the age of 16..He is a bit agressive..Both the parents love their Son atmost..They give lot of time to him.The problem here is with their 6 yrs old kid..Sometimes he is behaving so odd..He is feeling so happy by hurting others..he beats other kids or scold them with dirty language..whenever we say its bad he is saying he is feeling so happy if he do that. for 1 minute he is beating her mom and dad and next minute he immediatly saying sorry.