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.