If you have a positive, loving household, are a good mom, and have always firmly disciplined him for disrespect (since first snide comments which usually are attempted around age 3 with most kids) so he knows it is absolutely not tolerated, then he may have a more serious issue.
If he has ever gotten off lightly with mild consequences, being ignored, or received talks about your "feelings" for saying something mean to you, it's normal. Kids tend to lash out at parents who enable it. Especially if they say their "feelings are hurt". It's an attention and power thing that gets worse with no effective deterrent. Disrespect to a parent is a capital offense in our house. If a child doesn't respect his mom, he won't respect others or himself. You have to treat him with respect and love (which I'm sure you do) and crack down big time when he does that after being warned calmly. Maximum discipline.
This book is fantastic for building respectful, kind kids in his age range. My 3 year old son likes to say something mean in a joking way, and when I make a pretend shocked face, he says, "Just joking mom, I love you"., but my 5 year old wont' even joke around and say mean things to me, she is so sensitive to people's feelings and sweet.
"Back to Basics Discipline" by Janet C. Matson