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.