It is probably hip dysplacia and/or arthritis, all the big breeds tend to suffer from them as they age. Some type of gangplank or "U" shaped piece that he can use as a ramp, with two pins or ties to fix it into the companionway, might be necessary.
There are doggie boarding ramps sold for waterdogs to get back into duckboats, one of those might work up. Some fold to stow as well.
I wonder if he hurt himself and now won't repeat what hurt. It may be a good idea to have your vet or at least a vet check him out if you are traveling. He does sound a bit heavy at 100#. If he is overweight a diet is absolutely a good first step.
What I do to manage he companionway is a ramp. My food motivated dog had it mastered in 10 minutes with proper treat motivation. This is the ramp I use: http://www.discountramps.com/pet_step_dog_ramp.htm