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Dont do it, unless you are going to be responsible for his mistakes. You're score could even drop!
He is 18, I would talk to him about credit, and have him do it on his own. Send him links to these sites. He is an adult and needs to take care of himself.
I would not AU him on cards. Let him start small and work his way up. If you AU him on cards and his history/score gets too high too quick he may end up with a bunch of offers fo high limit cards and find himslef quickly in trouble. It is easy enough for a college age kid with a job to get one or two small cards and build from there. Also, it limits your liability and teaches him better money skills. The key is to not jump start their credit, but to talk to them about proper credit handling.
Another option is to start them with a secured credit card right away so they can not exceed the balance or rack up major debt while learning how to handle the debt.
IMO you are correct in adding him as an *AU*. This starts his credit file and when he has a job and some savings then he can apply for a CC in his name.
I stsrted my sons as an AU to 2 of my CCs when they were 16. At age 18 they received offers from these CCC and then just applied to who they wanted.
As an AU they can use the CC and I made them pay me every month. This way they get into the habit of must pay every month.