For
 the past 6 months, my cat has been howling non stop. He isn t injured 
nor sick. He did it occasionally before his mama died but now he does it
 constantly. Any reason why he does this?
Update: 
    He's fixed and is an in door cat
Answer
Ace:
 Normally when they howl constantly its a mating call.
Raya:
 maybe it wants to be trnas-spiecies