First time traveler

I would take him to your own bank and see what option is best. I know there are much more features available than when I was a teen and I would want to take full advantage. As for travel, you want a card that does not have a foreign transaction fee.
Any card type will be fine, but you should get one that has a readable chip.
When he withdraws cash, he should select Euros and use bank affiliated ATMs.

I know this isn’t the same thing as him having his own account, but do you have a credit card account that would work, and might make him an authorized user for now? Would that be easier under the circumstances?

Which credit cards do you business with? Visa? Mastercard? Call them and ask them about obtaining a card for him to use while traveling.

Visa and Mastercard are most used and accepted in Germany. Amex is less accepted (high fees).

In tourist areas you will find over 90% of businesses accepting credit cards - the number is lower in less touristy areas. Some places accept credit cards only from purchase sums above 5 or 10 EUR.

By the way, you are of legal age in Germany at the age of 18.

Good advice already posted, visa or MC works.

I do suggest him getting a card from the same bank or credit union that he has a checking and/or savings account with.

I have Capital One Visa card. I made my 19 year old nephew an authorized user and had credit card issued in his name linked to my account. This particular card charges no international transaction fee and is also contactless, something we used a lot for transit and other expenses. His expenses were tallied separately but billed to me (he paid me back).

Your son will also need a separate ATM/debit card to get cash if he needs it, he should NOT use the Visa (or Mastercard) for cash as that is considered a cash advance and there is usually a hefty fee for that. If he doesn't have a debit card, your local bank/credit union should be able to set him up. Just make sure he knows his PIN number.

There are also other cards that are available for people with low/no credit. I find NerdWallet a good place to research options. Not sure if you have enough time to go through that process though.

The first credit cards our offspring got were from the local credit union. My first card - from Bank of America, more than 274 years ago - was a Visa designed to appeal to college students.

Whatever card you choose, make sure to consider the credit level... Will he need thousands in credit or hundreds?

Agree with much of what was posted, but another option is to simply get him a card linked to an account you already have (MC or Visa) and have him added as an authorized user.

Getting him is own account is also a good idea, but if he can take two credit cards, one to use, one for back-up, then that would be best.

As others mentioned, he also needs an ATM card for cash withdrawals, especially in Germany, and visiting friends who might be going to local spots, he will need some cash.

Might be a good idea for both you and him to read through this: https://www.ricksteves.com/travel-tips/money/cash-tips

Get a credit card with no foreign transaction fees and an ATM card that reimburses bank fees. Ensure he knows to always pay in the local currency, do not have the vendor convert the charge to dollars. Use the wallet feature on his phone to tap rather than taking his card out of his wallet whenever possible. And starting now as practice, begin carring his wallet in his front pocket.

BTW, congratulations to you for raising a child that you trust on their own on this trip.