We have an 8 year and 16 year old to amuse on long trips in the van, we tried games to play but game turned into competition and fall- outs so they stopped. The DVD player was excellent but I am guessing he is going to be riding shot gun with you and not at the table so would be a bit difficult.
Our solution was to buy them a Game Boy each, yes I know not everyones solution but we are not bored and 8 and I remember having Bunty and Jackie in the back of the car as field upon field didn't interest me at 8 either (much to my fathers disgust). We got them on Ebay along with many cheap games as the DS has now taken its place (they got those for christmas when we saw how much pleasure they got from the original ones). I also have to add how much my daughters reading and thirst for books has improved since she has to read the instructions on the screen or doesn't get to play it properly.
They are only allowed them when we are on the move and they live in a tin the rest of the time and never come into the house, it keeps them a novelty. The children have now got used to looking straight out of their window the minute they hear me shout "water or church or other things of interest" as they have missed too many things by not looking up quick enough, and it really pees them if the other sees something and they don't .
Have a great trip I bet you are both going to love it. We stay on Aires most of the time which the 8 year old loves, 16 year old likes the sites for the totty that he might find (apparently sparse on Aires)