That's an arguable point, but in the end it comes down to personal taste, because you can get the sound you need out of either instrument.

The electric bass is perfectly fine for contras(though an upright bass or an acoustic/electric may look more folksy)--if your son is an electric bass player and he moves to an upright, he will have to relearn what he knows now, and he'll end up carting around a large and bulky instrument. That, however, would be my preference, his may be different.

what is important is that he learns how to play the bass part that is appropriate to the music on his instrument. It is also important for him to know that the dancers always follow the bass.