got an email from someone claiming to have a patent pending on the product I was selling, as well as claiming he was trying to have the US border protection stop the import of these products.
Patent pending tells you very little. He may have a provisional application which itself cannot mature into an enforceable patent. He may have a pending utility patent application which can mature into an enforceable patent - that is, if everything goes "right" [from the applicant's point of view].
He would need an enforceable *patent*, not an application for patent, in order to attempt to get "the US border protection stop the import of these products".
Is there any way to find out what his patent covers before it is published?
That information is not public until the application publishes or the patent issues. Therefore, the only information you can obtain about the application is information that the applicant gives it to you. The applicant can share as much or as little as he wants.
Note that even if the applicant shared all the details of the patent application, no one can know what the patent covers until the patent issues. Until then, the claims are in flux, in negotiation with the patent office.
Internal Chip LEDs have been around for years before his patent application. Is the coding on these LEDs (that would change how they blink/flash) patentable? Would the trail of light be patentable?
The only valid answer to your question is an LED feature is patentable if it's novel and non-obvious. Making such a determination is a complicated and very fact specific analysis. You don't have enough facts.
Another product from years before his came out left a trail that was visually identical.
If that's true, then generally speaking the applicant is not legally entitled to obtain a patent on the particular trail-of-light feature embodied in the old product. Of course, the Patent Office may not know about the older product, and so the patent might issue anyway, in which case this is an issue which might be used to invalidate the patent in litigation.