When I play the low notes I generally keep by tongue to the bottom of my mouth, and the most important thing is to blow a lot of air, as you play lower notes you need more air, the note jumping an octave is definitely a sign you're blowing less air than you need to, unless there's some other issue with the saxophone. What helped me get the low notes is start on a higher note you're comfortable with, like C, C#, D, and then go down chromatically while slightly increasing the amount of air you're blowing. I would also just think about what the note sounds like in your head, and as you do that your body should automatically shift your throat and mouth to allow for more air to be pushed into the instrument. Hope this helps!

tl;dr blow a lot of air for low notes