Well, my girlfriend has been calling my landlord for the past couple of weeks and getting no response. Since I started leasing I have had a few complaints and it takes many calls before I get any action. Now the refrigerator leaks, the a/c thermostat is out of whack and not cooling properly, the grass needs to be cut and there is algae on the steps (I live in upstairs of a duplex). My girlfriend and I have both slipped on the steps (even while holding on the rail) after it has rained or in the early morning with the dew. My girlfriend had a big bruise on her butt. The steps are the biggest issue because my mom sometimes visits and she has severe neck trauma and this could turn out to be very bad if she fell. It could result in death because her neck is so unstable.

Can I withhold rent or should I send a certified letter? I need immediate action, especially to get the steps pressure washed. Also, his mother is the owner so I only have her address. I don't have his to send the letter.
Additional Details
His mother (landlord) lives in Florida and he (the guy/son who acts like the landlord) lives here in Georgia.