well. my family (husband, 2 1/2 y.o. and me) went on a long road trip last summer. honestly, it was great. we had a dvd player we didn't even use till the way home. it was a great adventure! he had a little $5 travel magna doodle which kept him occupied for quite awhile. LOTS of snacks, books, cars. and yes, be prepared for at least one or two lengthy stops. he has got to stretch his legs and run some energy off. it really isn't that bad. does your son do well in a car, in general? i think mine didn't realize exactly what was going on - for the first several hours i think he was just looking at it like any other car ride. hmm. where we going. wow there's a bulldozer! woa, look, cows! eventually he took a nap. (we started EARLY in the morning, in order to let him sleep through a lot of it. it worked!) your bf is going to have to be patient and roll with the punches. just like anything else, you have to take little man's cues. but i was very pleasantly surprised by how well my son did. it's a tough age! but like you we really needed it, so we went for it. and never looked back. don't stress yourself out over it, let yourself get caugh up in the excitement and make it positive. it'll be a blast! have fun!
ps, someone mentioned sitting next to your son. since it was only the three of us, i sat my son in the middle of the back seat, surrounded by his "entertainment" on either side (blankets, toys, books etc) so that he could reach everything. then i put my seat as far back as i could, and my husband's legs are long anyway, so it was almost like having him right between us. worked great!