Try to line up lots of help with childcare for around test time.
Find a good school for your son, one that will not keep you worrying while you are in class or studying. A school that will really work with your son so that you will see his progress will do wonders to make you feel less guilty for not being able to devote the time to teach him stuff.
Make the time to be with your son, other than while you are providing for his basic needs. It doesn't have to be a lot. Even if just half an hour before his bedtime can be a time you both will cherish.
Speaking of bedtime, plan on doing a lot of studying after he has gone to bed.
Don't expect to be able to be very involved in extracurricular activities. Remember that something always has to give. Study groups can also be difficult to juggle when you have a child, so try to find people who understand your constrains.
Try to set realistic goals in terms of academic performance ahead of time. You will NOT be at the top of your class as a mother, and definitively not as a single mother. You just cannot compete with classmates who have no other responsibility at the moment besides medical school. You can get very good grades, but setting your goals to be at the very top of the class will just cause you stress,feelings of guilt over the enormous time this requires away from your son, and it is probably not very feasible, under the circumstances.
Expect to be VERY VERY tired, particularly during the first year.
Good luck!