In this context, the father is always calm, so it is not a changing condition. Then, could we use "ser" instead of "estar" too? "Meu pai é sempre calmo"?
Yes, it also shows this condition lasts longer, while 'estar' can be understood for a long period, but not necessarily. Also, using "ser" means it is a personality trait. For example, if you say "meu pai está sempre nervoso" it may also mean he is nervous during all day long.