It's sad that this is before Victorian, because those fellows could amputate well and quickly--we are talking half a minute to 2 minutes for the really practiced ones. Pre-Elizabethan doctors were not so well versed in the art of amputation, because the autopsy craze hadn't taken hold, and that's where a lot of Victorians learned how--on cadavers. During the Pre-Elizabethan times, there would be less opportunity to practice such a thing.
Let's take your second question:
How would it effect their social standing?
Depends on several factors. If they were injured in war--or if the story of that injury is a good one rather than a mundane one (I tripped on the stairs and it got infected vs. epic fight with a boar. It took my leg, but I ate it. Vengeance is savory!) Injury in war can actually raise their standing because they gave to their country.
However, it means that they will be looking for ways to be useful in battle. They'd want to do mounted combat, of course, and will want to be in battle to prove themselves.
Since this isn't a birth defect, it might not harm their standing at all, as long as they produce heirs and or otherwise upstanding. If they are not upstanding (no pun intended) then this weakness will be used to further denigrate their reputation.
Socially it would hurt them for activities involving walking, though they may work to be able to do those just as easily.
Now let's look at question 1:
What would be available to this fellow to replace the leg.
That's going to greatly depend on finding a craftsmen. Today prosthetics are many and varied, but they often come from the same manufacturers, but back then, you would engage a specific craftsmen to do the work.
This is going to be limited by the fact that amputations that the person actually lived through were relatively RARE, because there's more to it than simply lopping off the limb--you've got to tie off blood vessels, leave extra skin to close it up, be careful where you cut. At the time you are talking about, most would be dead.
So because it's rare, there aren't likely to be people who are specialists in prosthetics. During America's Civil War for instance, there were enough amputees that a specialist business grew up out of it, but your noble is more likely to hire a craftsmen they know, or send for one at great expense who has been known to do such a job in the past.
Materials used, and the people hired include woodworkers, armorers and leather workers. There's evidence that wood and metal might be used together in order to fit on to the stump.
If you are wondering about a hinge or anything--while it could be possible, during this era it's not common. Possible, if the person in question pays a lot and gets the most advanced thing. Here's a snippet on the most advanced during that time period:
In 1508, German mercenary Gotz von Berlichingen had a pair of
technologically advanced iron hands made after he lost his right arm
in the Battle of Landshut. The hands could be manipulated by setting
them with the natural hand and moved by relaxing a series of releases
and springs while being suspended with leather straps.
Around 1512, an Italian surgeon traveling in Asia recorded
observations of a bilateral upper extremity amputee who was able to
remove his hat, open his purse, and sign his name. Another story
surfaced about a silver arm that was made for Admiral Barbarossa, who
fought the Spaniards in Bougie, Algeria, for a Turkish sultan.
History Prosthetics 04Mid- to late 1500s
French Army barber/surgeon Ambroise Paré...invented an above-knee device that was a kneeling
peg leg and foot prosthesis that had a fixed position, adjustable
harness, knee lock control and other engineering features that are
used in today’s devices. His work showed the first true understanding
of how a prosthesis should function. A colleague of Paré’s, Lorrain, a
French locksmith, offered one of the most important contributions to
the field when he used leather, paper and glue in place of heavy iron
in making a prosthesis. SOURCE
As you can see springs and hinges were possible for knee joint action and this gives you an idea of the materials used.
EDIT: By Knee Joint action, I just mean that the knee joint can be adjusted manually. It would not actually work like a regular knee during this time period, although there might be an adjustable hinge.