If a physical illness had left your ds feeling the way he does, @Oopupsideyourhead, you wouldn't have had a moment's indecision about keeping him off school, and his mental health is no different, in my opinion.
I suspect he will be better able to cope at school, having had a bit of time to recover.
And whilst it is important to learn resilience, and how to cope with pressure and stress, it is also important to learn that, if you reach breaking point, it IS ok to take some time off. Poor mental health is real, and is just as much of an issue as poor physical health - but we are expected to just soldier on, if our mental health is suffering, whereas it is far more acceptable to take time off with physical illness.
People should not feel any shame for knowing their limits - because when someone is pushed beyond those limits, the effects on their mental health can be severe, and long lasting. I have friends who have been forced way past their limits at work and have, as a result, developed depression and anxiety that has meant they have not been able to work since! Surely it is better to take a short amount of time off, and recover one's equilibrium, than to end up with life-long poor mental health, and unable to work at all!!
Sorry for the rant, but as a sufferer of depression and anxiety, it really angers me when people dismiss the need for others to care for their own mental health!