I'm familiar with POTS but don't have it myself. Here's what I would choose:
Salty snacks- pretzels, posh crisps, salty popcorn etc.
A mini fan that plugs in to your phone charger slot on your phone. So handy when you have a hot spell (which I do get!).
Does he have a decent water bottle? Comfy size for carrying around but with enough to keep him hydrated.
I have a lovely heated throw on my sofa that is so cozy and helps me keep warm when I'm struggling.
Electrolyte drinks/powders/effervescent anything to keep your DH comfy as it can feel terribly miserable when newly diagnosed, feeling like shit but haven't yet worked out how to help ease the symptoms.
Most of all, do something to let your DH know that you 'get' it. Empathy is key. I've never felt my DH has 'got' it, not for one moment. He gets better with experience but still lacks the understanding side of it. Shoe him your support with kindness and love.
Wishing you both a merry Christmas. 