I would call his pediatrician or on call advice nurse. You can give him Infants Tylenol to help relieve any uncomfort that he is feeling or maybe some Children's Robitussin (or something similar to it) just for the cough and stuffy nose. But definitely check with his ped or nurse first.
To help with the stuffy nose, you can use a saline nasal spray to help dry it up. Or you can use a humidifier in his room. And you can turn the shower on hot, let the bathroom get steamy, and then sit with him on the toilet for about 15 minutes to let the steam clear out his congestion and stuffiness. I will also rub a little Vicks Vapo Rub onto the outside front part of their PJ's or sleepers before they go to bed at night to help with congestion. Just a small dab so they can't accidentally rub it into their eyes.
He could also be teething, as those symptoms can also appear when they are teething.
Both of my kids illnesses come on like that, all of a sudden too. I think it is just mainly that they are too young to be able to tell us that they are feeling ickey beforehand so we only notice it when it is full blown.
Going back to the medications....be sure to check your labels....often times they will put aspartame in them to sweeten the taste.