Your dog shouldn't have a problem getting up and down the
companionway stairs (unless he is old) ---suggest you just place a rubber-backed carpet or rug at the base of the stairs so he has some traction to jump up and a non-skid landing pad when he jumps down and you should have no problem
Additional suggestion to make life easier - put a chest harness on him tied to a short lead that you can clip onto your jack lines with a carabiner---that way your pooch can roam forward at his leisure (for a better view, to catch the
wind or to do his business or whatever) without you worrying--netting on the
lifelines are also required (saves you from having to
lift him back on board) because
dogs can slide off without the netting. Thedoggie life-jackets are a pain in the butt, uncomfortable and hot and really are only necessary if you are going in the dink in rough
weather.
Just remember to keep your dog hydrated (use a non-spill "Water-Hole" watering dish)
Dogs are really adaptable... I wouldn't worry too much about it ...just take your 'best friend' on board and enjoy