8 days in Italy

Are you still going to Germany? If so, before or after Italy? Reason I ask is because some people are going to talk to you about jet lag......It might be less of an issue if you have visited Germany first. Also, do the 8 days include your arrival and departure dates? If so, that effectively makes it 6, 6.5 or 7 days.
I'll be honest, I never took our teens on international travel, but where to go depends a lot on what you like to do....and how much tolerance your son has for art and old stuff.

Depending on interests and when you are travelling, Venice holds a Biennale every year alternating between art/dance and architecture. 2020 is the architecture year. It is global event over numerous months so not your usual first time visitor thing, but I find 17 YO boys have short attention spans...so this is something different. For the same reason, I would question what Florence has to offer. I appreciate that Florence is renowned but we do need to leave places for our kids to explore on their own later. I might suggest Bologna. It is on the train route between Venice and Rome and is a university town so may be more age appealing.
Further details may provide other options and the better ability to suggest how to divide your time.

Since you're coming from Berlin, you might(shudder) be able to get away with 2 nights in Venice. Then 2 in Florence and 4 in Rome. All is however, dependent on what you plan on visiting in each city. 3-2-3 may work better for you.

Make lists (both of you) of what you want to see, then allot days. There is no one size fits all!

Yes, we are doing Berlin first so hopefully, we will be up and running by the time we get to Venice. Our flight arrives in Venice at 7 pm though. I was considering going to either the Amalfi Coast or Cinque Terre. Which do you think makes the most sense. We are big walkers and might go to a few museums but prefer to take in the culture by foot. Hope this helps describe us as travelers. Also, if you have any hotel or neighborhood recommendations that would be great. We don't need fancy accommodations but would like to be able to walk to places from where we stay. I haven't been in Europe for over 19 years--I can't wait to explore it with my oldest child.

I forgot to mention we will be there late March/early April.

Eight days would already not be enough for Venice, Florence, and Rome, so adding Cinque Terre would not be advisable. It takes an awfully long time to get there--see the schedules on the Trenitalia website before deciding.
Also, that time of year it is common to get rain--make any plans for CT able to be cancelled.

A suggestion for Venice.
https://www.venicekayak.com/

Really worth a look.

There's much more to see and do in each of those places than you can possibly do in 8 days, no matter how you divide the time. It's so subjective, my best advice is for both of you to read a little about the sights in each (guide book and/or internet) and make a short list of priorities. Then we can help you determine what is feasible within your time frame.

Venice doesn't have "major" sights, the city itself is a joy to be in for those that love her (most people) and away from the daytripping crowds, it's quiet. Florence is an art lover's eden, but there's more to it than painting and sculpture. Both are very walkable. Rome is a big, bustling city with many major sights and so many "minor" ones that you couldn't do them justice in 8 days, so you must pick and choose.

For 8 days, pick two places and enjoy your visit. Don't waste so much time traveling. Rome is good for 5 days and Venice for 3.

With 8 days, I'd do Venice and Florence or Venice and Rome, but not all three.

For what it's worth, my 20 yr ish boys were bored in Venice after a day - enjoyed the sights, walking the canals, a little shopping, but quickly became bored after soaking in the awe of it all. Worth a stop - we spent one night and half the next day before moving on to Florence which they enjoyed more. But even then....They enjoyed seeing The David, the Duomo, and shopping areas, but not a lot to hold their attention after a day. They enjoyed Rome much more - the chaos of it all intrigued them. More to see and do and much more adventurous for them. If you were to ask them today which would they pick to go back to they would definitely say Rome. In fact I think they have been back 3-4 times now.

We went to Amalfi from Rome - took train to Salerno then the ferry to Positano for the night. Really enjoyed that a lot. Again, not a lot to do for a 17-year old, as it's not "beach" season, but the cool factor will be a nice change. To return to Rome you could ferry or bus over to Sorrento and train back up to Rome or the easier way is back to Salerno to Rome. You have to really want to see Amalfi to take time away from Rome and the other sights. It's not a quick trip at all. At 17, and not knowing what his interests are, I would imagine his interest level is not going to be the same as yours. While you may enjoy cafe's and wine, what are his interests? Is he in to Museums? Art? I have been with friends travelling from Venice to Rome, and we stopped in Florence for 1/2 day just to see the sights, as they had no interest in really anything other than seeing the city, the David, and the Duomo which was all easily done. I have also spent overnights as well. Perhaps more time in Rome would give you the opportunity to do some day trips - ie Naples, Pompeii, Orvieto. The night tours in Rome are fun seeing the landmarks all lit up. The Vatican at night is lovely. Much much more in Rome to keep a teen occupied. Good luck -