Black ink tattoos are not that hard to remove these days. Depends a bit on the tattoo, the location, and certain skin qualities, but the technology is pretty advanced. Yes, removal costs more than getting the tattoo in the first place. But it's not like prohibitively expensive -- maybe around 1k for a small-to-mid-size black ink tattoo. It can take a few years. But it's doable.
Pete Davidson is in the process of removing something like 200 tattoos all over his body, and you can see how dramatic the transformation is:
https://www.instagram.com/reels/DF5_4v4vt7f/
I'm sure close up you can still see some shadows from some of them (especially the color ink) but it's really not that noticeable).
I say all this because I see a lot of people in this thread freaking out about tattoos and acting like they are irreversible mistakes. They aren't. You can get an ill-advised tattoo at 18 and get it removed at 25 for 1-2k, and it's like it never happened. The cost of removal is high enough that it will deter future tattoos for people who regretted past ones, but not so high to prevent someone from doing removal.
Also the lasers they use to remove tattoos seriously get better all the time. The technology has advanced a ton in just the last 10 years and will likely only get better, because tattoo removal has become a big business and being able to promise faster, better removal is a big selling point.
An ill advised tattoo is so far from the worst thing an 18 year old could do, I would honestly count your blessings that this is what his rebellion amounts to -- an ugly but reversible physical adornment that he got primarily to annoy you (and it's working!).