It was 30 years ago that I used the tampon method, then my next Doberman wast the one I have now & I used the backer rod as shown in the video that
@JanS linked for you. Also the size of tampon or backer rod is dependent on going deep to hold it firm. Too big and it won't seat down in the ear canal, then wobble and come loose. Too small and it will go deep but then wobble around and cause sores and irritation. My pup was cropped at 8 1/2 weeks, I brought her home at 10 weeks after stitches were out and posted constantly until almost 6 months of age. Usually she was in posts with no more than 30 minutes out while her ears dried and aired out when I reposted every 3 days. It looks like you have a medium crop which usually is easier to stand, but I wouldn't start testing them until all his permanent teeth are all in and his canine teeth are of good size. The calcium that the body takes to grow that mouth full of teeth will slow down the ears stiffening enough to hold and that's why they'll look good at 4 months, the droop after a few weeks. That will set you back. And as you mentioned, the ear leather itself is different for each dog, some being faster to stay up than others. Best of luck, keep working at it, try some different methods/sizes so he's comfortable. It will be well worth your time and really, you are almost there!