What is the word for "stepping stone" if it is used in a derogatory sense?

You might say that He used him as little more than another rung on the ladder to success.

I wonder why you can't use stepping stone; after all, the Monkees did:

When I first met you girl you didn't have no shoes
Now you're walking 'round like you're front page news
You've been awful careful 'bout the friends you choose
But you won't find my name in your book of Who's Who.

I said,
I I I I I'm not your stepping stone

A stepping stone can be used for progress in a neutral way (as in, the minor leagues were a stepping stone for the star third baseman), or it can be used with a spirit of maliciousness or selfishness, as in your example. When the person using the stepping stone does so in a self-serving way, I think you can use context to convey that negativity, without changing the expression stepping stone.

For example, you might be able to use a word like "just" to help express this sentiment. Compare:

His time in college was a stepping stone to his pro career.

with:

His time in college was just a stepping stone to his pro career.

In the latter sentence, the word just trivializes the college experience, while in the former sentence, college seems to be more valued as a developmental step.

It's not a negative or derogatory sense, so much as it is derogatory to say of someone that they used another person as "a means of progress", and nothing else.

The first person had a relationship with the second purely because doing so was a means of progress for the first person's career. Once the second person had served that purpose, the first person, having no real feelings of friendship for him, was not willing to give him the help he needed.

I wonder whether you aren’t looking for something like training wheels or a crutch. Those are not quite the same as a mean to an end, but have some derogatory component.