Hey M.! My daughter was 7 1/2 months when we moved here from Mississippi six months ago - it was a 12 hr. drive for us. We split it up over 2 days but we drove about 7 1/2 - 8 hrs. that first day. She was a breastfed baby so we made MANY stops along the way for feedings & diaper changes and we had a 4 yr. old with us too.
Like you, I packed a diaper bag full of stuff I would need to have close by (diapers, changing pads, lots of wipes, clothes, blankets, pacifiers, bottles - unless you nurse) - I had to change my daughter in our van many times, mostly on my lap so a changing pad really helped.
A TOY BAG was something else I brought along that helped - I packed books, shaker toys, rattles, etc. - each time she would get a little fussy I would offer her a different toy to play with for variation. And don't forget a few snacks too (like cheerios if he eats them) Oh, we also had a dvd player for our kids (a dual system) so that helped out too b/c I would play Sesame Street videos, anything with kiddie music, etc. to help distract from the long trip.
She slept a lot too. We left early that morning so she was able to take a couple of naps along the way & this helped a lot. Pillows & blankets for you would be an added bonus for when you want to curl up and take a nap with him. Don't forget snacks for you too!
If you're able to sit beside him on the trip this would help too. This would probably make him feel more at ease with being in the car for such a long time. Just make little stops along the way, about every 2 hrs. and the trip should go just fine for you guys. You mentioned that he is a very good baby so this is a big plus, especially if he rides well! Just give yourselves around 8 hrs. for this 5 hr. tip (for stops to eat & rest) Hope this helps - I hope your vacation is GREAT!! :)