While I have personally never tried either fresh or dried valerian root on my cats, catnip company Meowijuana, among many others, offers a blend of catnip and dried valerian root, which they advertise as offering "calm and collectiveness" and "tranquility." The reviews are primarily positive, mentioning a mix of both "laying around" and "racing around"--catnip will affect cats differently, with some deciding to "chill out" and others being highly energized after exposure. It appears that valerian has a similar property, as CatHealth also suggests that valerian is a stimulant to cats, while UpgradeYourCat suggests it for calming and relaxing them. The latter recommends using "dried, finely minced root" cut into small pieces and offered in small quantities. All sources I've looked into recommend or sell dried root, with no mention of using fresh valerian, so you should have no issue using a dried version, but it should be pure valerian, not one formulated for human use.
As valerian, like catnip, can have a range of effects from "practically asleep" to "racing around at full speed" to aggression, and like catnip may not affect all cats, you may wish to start with a small amount of dried valerian to test his reaction. If you do not find much luck with valerian, or even if you do, you may benefit from additionally exploring silver vine and honeysuckle, as well as catnip itself; in all cases, you'll be using small amounts, with long breaks, to avoid desensitizing your cat to the compounds that trigger the responses you're looking for. Personally, I've had good experiences with a catnip/silver vine blend being extremely effective on my own cats, but like all blends, each cat has a slightly different reaction to the product. Some experimentation with the four main herbs that affect cats may lead you to the right solution to your cat's anxiety, and will likely be an enjoyable process for both you and him. You should always stay close by to observe your cat's reaction, and start small in case he does respond to any of them with aggression--a smaller dose will wear off more quickly, and you'll know not to try that herb again. A search for any of these single ingredients or blends should produce multiple sources available in your area; higher quality products will often cost a bit more, but may also provide a better response, so please consider quality and your cat's reaction alongside the price.