Has your DS started some preschool or similar?
My DD4 did something similar not with reading but with puzzels when she started the preschool at 2yr4mo. Before that she was all hot on them and doing very good progress - she had two 15-pieces jigsaw puzzels and could do them without any difficulty. And then, she started to ask me to do them and if I refused then she would want to play with something else.
I thought this was just a phase and did nothing.
She lost interest in them for a long time and really seemed to have forgotten how to do them.
She eventually became somewhat intetested again but now I have no clue how many pieces she is able to do.
By the way, she does have vision problems and according to the optometrist this is why she does not like puzzles as much, so it worth to check that as Grinity says.
But I am pretty certain that the preschool played a big part in it
