what bits of his routine are set in stone?
eg, when we go away (and we do, quite a lot) we take:
dd1's pillow and blanket, plus her bedtime story (Maisy's bedtime - has to be read every night).
a selection of current favourite books
a box of toys - some old favourites, some new (but variations on old favourites). we actually take the toys along packed in a Trunki suitcase (bought for the dds for holiday last yer) - this provides an extra ride on toy when necessary!
familiar food is a must for dd1. we mostly go self catering, as dd1 is gf/cf, but we have been half board before, and take along dd1's food (handy thermos - Lakeland sell a good one that is easy to fill up with rice/pasta)
if you are worried about his food, what kind of things do you know he will eat? is it possible to take those along? eg, when eating out with dd1, i would take along her meal (in thermos) but also know that she wouldn't eat much of it, so take along lots of other stuff that she thinks is a treat, but is actually halfway decent food - like dried fruit bars/Larabars, fruit/vegetable crisps (most supermarkets have a range of parsnip/beetroot crisps, and there are lunchbox ranges of fruit crisps now as well) oh, and there is a type of seed/nut bar (covered in carob) which dd1 ate for a while too - again, a "treat" to her, but actually quite healthy! found it in Sainsburys - think it was called 9 Bar.
will your ds eat thngs like nuts? a lot of children like them, and they are quite filling and full of nutrients, so you don't need to worry as much about what he is eating
I also facotr in daytrips where possible to things the dds like - wherever we go we visit an aquarium, as both dd are fish mad, and I know that is one day that we will all enjoy - or a zoo. these places tend ot be laid out in similar ways, so it is both new and familiar for dd1.