UGH!! A full sleeve??!! What on earth is he thinking? Maybe your older son can talk him into getting a smaller one somewhere that is easily hidden. He lives in Georgia for goodness sake! It's gonna be hot as hades and someday he WILL want to hide it if only for work purposes.
I'm sure he hasn't told you this yet because he knows deep down that it might be a mistake and a waste of money. I have a tattoo that I got when I was 20 and believe me, I wish I could erase it now

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Kids. What can we do but hope they think for a while before they do this type of thing. He does still live with you (at least part of the time) so I understand why this would bother you so much. He could be paying a portion of the rent instead. Don't feel guilty, K. None of what has happened in your lives is your fault. Remember that they are grown now and you have done everything possible to make their lives easier.