Ugh, so sad. I'm sorry to hear this. Personally, I would not take the truck for the tattoo. However, the drinking is another matter. If you fear that he is drinking and driving and you have every right to say that you are not paying for the truck anymore or insurance and his dad can now flip that bill. Talk to his dad and let him know.
He's legally an adult and I don't know what his future plans are-- college? You could hold off from doing anything and just get him through school and when he graduates, deem him an adult. No more help with truck payment, he's off your insurance, etc. If he cleans up his act, you can help again. If not, then don't.
I will say that plenty of kids do drink and smoke in high school. It is not ideal but most come out on the other side and wind up being upstanding citizens down the road. We rent a house to college students and the collection of whisky bottles they have going on the window sill I'm sure warms their mother's hearts. But they are all on track to graduate and I gather will someday soon be independent adults.
Again, I don't condone teens drinking and smoking. But, with so many who do that, I just wanted to mention that it does not mean he'll end up being some loser who never gets a job down the road. good luck
Sit back and wait. His romance with his father may be short lived. In any case the die is cast. Your son is an adult and his character pretty well formed. It is for him to decide which path he will take.
He is 18 and can legally do a lot of stuff without your permission. However, you are not obligated to pay any thing for him. I take it dear old dad is not providing any child support? Maybe its time he started paying for a few things since the boy is now under his roof. Really, the only major thing you had over the child was room and board and that is now gone. You are really stuck between a rock and a hard place. I am sure you want him to finish school. You could always take the truck back until he graduates. But, I've got a feeling that you are going to have a hard time getting the truck back.
And I am not sure what it would accomplish at this time. It would be better to save that threat for something coming up rather then for the tattoo. I would tell dear old dad that if his son gets a DUI on his watch that dad is gonna pay for it - and that is really expensive. Good luck