why is he in bad term with his father ?

Father for some reasons took away most of his mana power.
To be more correct, he took his mana back from Skelly.

Based on the raws I assume/ theorize that his father/Demon king wanted to have his own off-spring so he created a living skeleton by storing lots of his mana in it.
In the past he uncontrollably ate anyone due to his great hunger.
And because of that he has become very wary.

wow thanks !!I will try to see the raw too!

Meant to like.