Yeah, prepare for all possible behavioral emergencies, definitely plan snacks, get new toys to surprise him with, do everything possible to keep your regular schedule of naps, and set your expectations really, really low. Prepare yourself for it to, well, suck. Talk through all this in detail with your husband, so that you're both on the same page, you both have plans and strategies in place, take turns being the driver so that you can each get a break from your son sometimes, and make sure both of you have appropriate expectations for how the trip is going to go.
That said, here's some things that we found helpful in driving from NYC to MN several times over the years.
Since you only have one child, plan on you or your husband to spend all his "awake time" in the back seat with him. Interaction - whether reading a book or exploring a new toy or watching what is going past out the window - will help him a lot.
Breaks, breaks, breaks. Is there any way you can make the trip in two days? Think about getting a hotel one night - a little bit of extra money, a lot of extra sanity. A suite (where your son can go to sleep at his regular bedtime in a separate room) is ideal, but even if you and your husband have to spend the evening checking email on the bathroom floor, a good night's sleep for your son might be worth it. With the number of breaks that a young kid needs, and the amount of time it takes for those breaks (it will never be less than half an hour) - 15 hours of driving can easily turn into 20.
If your son has trouble sleeping in the car, consider rigging up a little "tent" for him. We figured out that if we hung lightweight blankets on the window, on the front passenger seat headrest, and next to him (in the middle of the backseat), it made it much easier for him to fall asleep. It wasn't dark or anything, but it did eliminate the visual stimulation of cars and trucks going by, etc. I'd sit in the back seat, read stories (as per our usual routine), then hang up his "curtains", my husband would turn on a lullaby CD, and I would close my eyes and "go to sleep." He would peek out sometimes to check on me, but when he saw that nothing exciting was happening outside, he gave up and went to sleep. Once he was out, I was free to read or listen to my ipod. Definitely plan your own grownup breaks so that you don't need to eat or use the restroom while he's sleeping - once he's asleep, you drive until it's time for him to wake up.
Good luck - he may surprise you! And one way or another, you will arrive at your destination. Drive safely!