I wouldn't use Dimetapp. That's potentially dangerous for his age. I know how you feel. I feel helpless when my boys are sick. My son is 24mos old. With just a cold, you should really let it run its course. Is he running a fever? If so, tylenol or motrin will be good. Otherwise, you'll have to let his body handle it. It's hard I know. He'll be ok though. I know you're in OH, but I am going to send you a link to my pedi's website and it actually has on there tips of what you can do with your children with a common cold and when to take them to the doc/ER. I hope this helps. Hope he feels better soon. :-)
http://www.cactuschildrens.com/web158/
The info is in the 'eye soring' blue print. oye...I'm going to have to email them and have them change that. It bothers my eyes.