I sent a design of mine to a t-shirt printer who really wanted to use the design. He promised to credit me for the design as he prints them (and sells them for profit), but has yet to meet those promises. He also detached my signature from the design and slightly skewed it. I am curious to know if you have any advice or suggestions for me. If you do, that would be much appreciated. Thanks!
Do you have documentation of the agreed terms?
I just have an email thread between us. He says several times he “would definitely give me credit.” There was no contract or money agreement made.
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Also, you can credit yourself. You have something in production, make a big deal about it via social media.
Also also, don’t do work for free. You wouldn’t be so bummed out if you got paid! 
Right! So, should I email him and tell him to stop printing them, or what? Ideally, I would like to be compensated and I don’t think it is fair for him to be profiting from my design when I am getting nothing, you know?
Put your design on your website as one of your sample designs.
document all about it on your site. Date of creation, current distribution, etc. document first time you used it, and how you came to arrive at this design.
show other various versions of your design, to show more input that you are the original creator.
You may also want to trademark your design.
If your design blows up, where it becomes very popular,…you will have better chance of laying claim.
Well said “KenoLeon”
Learn the lesson and move on. And be smart. Whenever you get deal with someone must write and compose a documentation that will help you to save yourselves of scamming.