Is it normal for a 9 week old baby to choke on his own saliva?

Yes. That is very normal. Babies produce a lot of saliva, and that is a good thing. A nine week old doesn't have a lot of control over swallowing and control of his tongue and other functions of the oral cavity other than suckling. Just be patient. He/she will grow out of it.
It can be. Babies are not very good at coughing effectively and getting mucous out of their throats. You may need to use the bulb to suction out their nose or throat when he is congested. Use Normal Saline drops to loosen the mucous. Use a cool mist humidifier to make it easier to get the mucous out. If your baby has more trouble breathing, see your doctor.