In your carry on make sure you have a change of clothes for all of you! If DS (or you) vomits or he (not you) has a nappy incident on the plane you will need this - or if your bags get lost in transit.

I would take the pram and no you will not get to chill out at the beach while DS plays happily as he will probably do a runner. He may actually hate the sand or be freaked out by the waves so don't be surprised if he doesn't want to hang out there. I would see if you can get a portable dvd player and some dvds of shows he loves and some books for him as well.