Hi K.!
I taught first grade for 8 years and in my school district, spelling wasn't formally taught until the latter half of first grade. Initially spelling isn't emphasized because the goal is encouraging kids to write. If they get bogged down with spelling every word correctly, they write less or become so frustrated that they don't write at all. From your examples, your son is doing very well with writing the sounds that he hears in words! I would be excited with a beginning first grader who spelled that well!
My son is also in first grade this year and we play the spelling game (in the car, in the shower, waiting in line, etc.) where we choose a word, he spells it, and then uses it in a sentence. We begin with easy words that are spelled like they sound and follow a pattern (ex. - at, cat, bat, sat, etc.). This shows him that if you can spell one of these words, by changing the beginning sound you can now spell many more words. This is very low key, no pressure, and if he makes a mistake, we spell it correctly for him and move on. He loves it because it is a game and not something we are forcing him to do.
So, encourage your son's ideas in writing and don't focus on the spelling. You want him to see writing as creative and fun, not as something he needs to worry about getting wrong.
Good luck!
K.