Explain his symptoms better, please.
I'm not too experienced with Gerbils, so I'll relate it to rats. They are fairly similar, afterall. I haven't heard of rats catching the typical strains of the cold from humans, but they can catch a few other strains of bacterial illnesses. Still, take precautions and in further cases of handling your gerbil while you are sick and be very sanitary (wash your hands & such). However, since rats can get pneumonia, I'm sure Gerbils can. Along with URIs. Both can appear like the rodent has a "cold".
Either way, I'm sure you expected us all to say this, but I advise you take him to a vet.
Good luck.