It's stapled in my passport so I never really know if they ever look at it when I hand over my passport. It is hard to know what advice to give about this. Taking a photo solves the problem of forgetting the number.
My last TM6 has been stapled into my passport for over a year. I took a photo of the TM6. If it ever got lost maybe that would help when I report the loss. It would certainly help if all I had to do was know the number.
In October I was expecting to have to wait for a one year statement but my Kasikorn branch gave me a one year statement on the spot. I suggest you ask your bank again.
If he is staying in a real hotel then that could be the issue. I am living in a condo. My TM30 was over a year old and filed by my landlord who sent me a photo of the receipt. I printed a copy of that photo. I have also traveled inside Thailand to Pattaya and Sisaket several times this past year.
The implicit message in my comment was also that when you hear stories like yours then you should note that information and plan to bring these things, in this case a rental contract, when you go to do your visa extension. It costs little to nothing to bring it and you have it when you need it. It is weird to me that people are always trying to find out EXACTLY what they need and at the same time resist (or complain) about bringing maybe more than they need.
Interesting. I used a TM30 receipt that was over a year old. I've never been asked to show a rental contract. However, because of experiences that you described above I always bring it with me.
Something else to check. In Bangkok you cannot apply until your insurance starts. So in your case, if you don't currently have insurance you would have to wait until Nov 21. If you do have insurance now I would recommend that you bring the insurance certificates for both this years and next years insurance policies.