I am a 2nd grade teacher, (on leave right now, have taught since 1991). Don't correct his spelling! If he asks for help, have him stretch the word slowly and write down the sounds/letters he hears. If he's a perfectionist, you definitely don't want to correct his spelling or he might be afraid to even try.
If and when he does start asking how to spell something, one technique I use with my 2nd graders is "Have a Go". When a child wants to know how to spell a word, I have them write the word 2 different ways. Then they bring the paper to me. Often times one way is the correct way. I ask them which one they think is correct and often they know. If neither way is correct, I write the "dictionary spelling" in the 3rd column and they they copy the correct spelling in the 4th column. (This is a page they keep in their writing folders, titled "Have a Go".)
I think it's amazing that your son is doing this! He's one sharp cookie!