Dappling is usually evident at birth, and may fade as the dog gets older, rather than the other way around. With a red I suppose it may be possible for the spots to become more evident as the true coat/guard hair (as opposed to puppy fuzz) grows in and/or the pet gets out in the sun more.
Dappling is caused by a (dominant) albino gene. Did you see the sire and/or dam? A pup absolutely cannot be dappled unless at least one of the parents is.
That having been said it is NOT wise to breed a dapple to a dapple, or a dapple to a red, (because sometimes the dappling on a red dachsie is very subtle) the idea is to avoid accidently breeding double-dappled pups as getting hit with that albino gene twice (one from each parent) plays havoc with eyes ears and occasionally innards. Some SERIOUS birth defects.

By chance, is he a red? Red dapples can be quite hard to see at various stages of life, and usually fade as the dog gets older. Dappling on a chocolate or black and tan dachshund, on the other hand, should be rather apparent from birth and doesn’t fade. A black and tan dapple is silver with black spots, and a chocolate and tan dapple is light beige with dark brown spots.

I have a black dapple. He was really dappled when I first got him. Then as he grew his dappling became less obvious. I think what you have is a Dachshund that was not meant to be dappled but has the genetic background somewhere. This happens and it is not unusual.

Enjoy watching what colors come out!

yes, some dapples are not born with very distinct spots and actually change as they get a little older. It’s usually around 8-10 weeks but I guess some could take a little longer for them to come out. Are they a few large spots or a bunch or small spots?