I feel like a failure as a dad

My girlfriend (25F) & I (25) have a five month old son together. My gf also has a seven year old son due to sexual assault. The seven year old has never had any type of male figure in his life prior to meeting me three years ago, as he was primarily raised by his grandmother, while his mom was in college. My gf got her son from his grandmother last year so he could live with us. He tends to give me a harder time than anyone else, despite me showing him the utmost respect. He’ll be disrespectful and flat out ignore me and it takes for me to threaten to tell his mom for me to straighten up. When he pushes my buttons, I’ll instantly yell at him. That alone will just make him cry. I feel like a failure because I feel he’ll never respect me like he should.What should I do? via /r/Parenting https://ift.tt/2omSp1b