Someone wakes up in a coffin, tries to punch a hole in it and gives up when it doesn't work... Because of the fact that he might be considered stupid.

I find it unbelievable that someone who on the one hand wants to escape this prison, on the other hand finds the leisure to think about what others think of him. The survival instinct would not have allowed such a thing to arise in the first place, don't you think?

Waking up in a dark box without knowing how one got there would trigger a shock and all one's energies would be concentrated on escaping. One would break one's fingernails and beat one's fists sore and tense one's thighs in order to escape.
So either the protagonist really tried to open the coffin by physical force or not. If not, then he is already delirious, but then your introduction would not be coherent.

Even if one had forgotten that he put himself in the coffin, the forgetting would only extend to the conscious, but not the subconscious level. ... Like, hmm.. finding your way home completely drunk without being able to remember it.

Panic is the opposite of awareness. A panic-stricken person does not think at the moment of panic that "his oxygen level must have dropped", he is merely the sufferer who feels that way. The author does this for the reader, of course, but the protagonist does not. I don't know of situation of stress in which I, as the stressed person, would be able to think about my biological processes. If I am, I am not stressed or in panic or shock. I can't be both at the same time.

Further, the man feels great, within a very short time he emerges from dilirium, does the realisation come to him that he slept super well? A bit too quick.

Any chance you could have done it more coherently?