"purchased some items on 2021 for 50 k total, stores them on a warehouse and sold a few for 20k."

Then there is no loss, yet. He has stock on hand. That is asset value, not expense as loss.

"So as of now he is a C Corp, no acceptance letter, I will file 2553 (late) with his first S Corp return."

Why? Why not just dissolve the C Corp for that one year with all 0? Forming an LLC and then trying to be a corporation would be different. Forming a corporation might show an intent, but the person did nothing as/for the corporation.