Could it be the time of day you're traveling? Maybe he's not tired, too tired? Not all bubs will sleep in their car seats. In terms of what to do for a long trip - does he have toys (soft or rattling) that he can play with? Can someone sit next to him (I'm assuming he's in the back) to help entertain him?
If he's sleeping fine at home but doesn't like his car seat, it may just be a phase. I'll go ahead and move your post over to the Activity boards where you might get some better ideas on how to help him like his carseat, since that seems to be the bigger issue, not so much sleeping.
hth
michelle