I am sure the thought of a 6 hour flight with a 19 month old sounds daunting! I have a dd also 19 months, so I know I would probably feel much the same way. Plus, for her there is no way a DVD player would do any good...she just has no interest in TV really of any kind except for the most fleeting of moments (the occasional 30 seconds). She does however enjoy books...the same books over and over and over. I think that is the good and bad part of this age...it is often something simple that will occupy them, but oh my the repetition.
Not certain what your little guy's schedule is like, but it would be hard for me to have dd on a flight for 6 hours that did not in some way involve sleep that would occur during her normal schedule. The white noise of a plane is powerful and has always induced sleep in my babies, so hopefully your fella will nap a nice portion of the flight. Plus, with a 6 hour flight there must be a meal and probably even a snack in his normal schedule. See just with those normal daily events, you already have some time used up
As for seat bound activities, I think simple is key...a small blanket or towel for peek a boo type games (my dd loves these), dampened q tips in a zip lock bag for painting with those pre painted books (this really saved us on a flight when older daughter was 2), a small blow up ball beach ball (Southwest had the whole plane playing with large ones on our last flight to FL), and maybe even a small bottle of bubbles (gently blowing just one or two at a time for the child to catch).
Hope your flight is smooth and pain free for both you and ds!