IMHO pedaling is something that simply comes with age. I got my son a trike for his 2 yr birthday and he NEVER figured it out all last summer. We finally put it away for the winter and when we got it out this spring, presto! Like magic, he could suddenly pedal.
For now, let him push himself around with his feet. You can also have him put his feet on the pedals and you can push the trike (this is a back breaker if you don't have one of those handles, though!) This will help him feel the recipricol motion that is required with his feet. Ultimately, the point of the trike is for him to have fun, so as long asn he is enjoying it, dont worry too much about how he moves it. He will figure it out himself over the course of the summer I bet.