Cancer's lucky item is silver. You can give him some silver jewelry. A 19-year-old boy has already formed a certain style. You should carefully choose the one that suits him. Cancer doesn't like things that can be bought in bulk from stores. I suggest some personalized items, such as necklaces or rings with names engraved on them, which will bring him good luck. And this kind of jewelry can be matched with anything, which is very suitable for otakus who are afraid of trouble.
In addition, crystals and pearls will also bring good luck to Cancer, and crystals will absorb radiation. I just don't know if you can find a suitable one for boys.
Cancer boys are soft-hearted and responsible, so it is also appropriate to give them living things. If you have a special relationship with this boy, you can give him a beautifully decorated fish tank and raise small fish. The two of you can take care of them together, which is very loving.