Speaking from my own experiences, I would recommend against sealing anything to the bottom unless you are an expert. Let me explain:
When I was setting up my first tank, I decided to use ceramic tiles, sealed with a silicone sealant (same stuff as what's used to seal the glass panels of the tank together) after a lot of research which indicated that these were the safest and most secure option (in this direction). I rinsed, soaked, and baked the tiles to remove any extra chemicals and to then get rid of the moisture. I watched all sorts of videos on how to apply th sealant properly and when I was done, I was really happy with it, perfectionist though I am.
About 9 months after setting everything up, I was moving and completely cleared out the tank, and was looking at the bottom. There was no water underneath the tiles, which was exactly as expected, but there was something much worse. I had used light-colored tiles, so I could clearly see tiny black dots of mildew. It wasn't much, but though everything was very well sealed, moisture had still infiltrated, and if it can get in, it can get out. You don't want those spores in your tank, no matter what.
Basically, I'm with your dad on this. I firmly believe I did everything I could have to avoid this situation and it happened anyway and I had to start all over. It was not fun.
You could try just not sealing it down, but you'll want to make sure that the pieces are too big/heavy to move in any current that might be generated (especially while cleaning/trying to catch your axolotl) because you don't want them to pinch together on his toes or anything. You'll also have to make sure that ammonia-filled air/water doesn't get trapped underneath and then cause problems if accidentally released/disturbed. Frankly, I would just go with sand, but it's up to you to weigh the pros and cons.
