If the child appears to be in pain, running a temperature, or is listless and sleepy all the time, take him to the doctors as soon as is humanly possible.
If there's no immediate need for medical attention, you can rest easy and make an appointment for the morning.
I am not in the medical profession, I am only a mother. My girl is 6yrs old now, but I remember well the time when I was on my own, with brand new baby, and not knowing a damn thing.
One thing I can suggest, that may give you peace of mind, and a professional to talk to: Do you have a HealthLine service in your area where you can ring up and talk to a nurse? They will ask you a whole bunch of questions about his behaviour and symptoms, etc. Then they will give you some idea of whether your son is in any immediate trouble or not. If you don't have such a service, maybe you could call the emergency line, and just tell them that there is no emergency but you would like some medical advice about what to do for your baby. I am confident they will help you, and talking to someone in the profession will ease your mind too.
Think back to what he has been given to eat or drink in the last few days. Is there anything out of the ordinary that he may have had, which could have caused the changes in his diapers? If not, he may be reacting to formula milk if he is on it. Or he may just be going through a phase.
Chat to whichever professionals you can get on the phone tonight, and go see your doctor in the morning if they feel like he should be examined.
Hope all this has been some help.