My son totally had the low frustration tolerance and confidence issues as well. He was comfortably reading chapter books before he would call himself a reader and he HATED stumbling over any word. If he saw a word he didn't know in any book, he'd say he couldn't read the whole book. He never "read" as a preschooler. Although he is easily, reading 5th to 6th grade level now in 1st grade. So I suspect there was some reading he could do before kindergarten, but that was just not where his interests as a 3 and 4 year old were. He much preferred to listen to a chapter of Roald Dahl than to try to read a "boring" early reader.
Anyway - I never pushed it as a preschooler. DS is my oldest and I wasn't looking for highly GT behavoir at all. I just followed his lead. Now my 2nd child, DD3 has some sight words and she more than happily calls herself a reader!
I'd follow up on it and look at his preschool enviroment if he has one, since he was willingly reading. But be willing to let it go for a while if it just seems like his temperment for a while.
