My 4mo old also has a cold. I daresay he got it from me, because I had symptoms first! The ped said the best thing I can do is continue breastfeeding so he can get the antibodies to help fight it off. I also take the suction bulb and gently suck out his nose to help break up the mucus from his sinuses. Make sure your little one doesn't have a fever, because that would warrant a visit to the doc.
Not much you can do but let it runs it's course. If he's congested at all, a humidifier will help with that. You can try suctioning his nose with the bulb syringe to clear it out but most babies hate that. You can't give a baby cold medicine so other than Tylenol or Motrin you don't really have many options. Hope he feels better soon.