*I really don't care for this new faddy 'Mental health day' phrase that's become so popular.
You've done the wrong thing imo OP, because what happens the next time he's too 'worn out' to go to school?
Learning to be resilient is a very important part of growing up. He needs to know that if he has issues at school, you're there to support him by way of contacting the school to help sort them out.
He doesn't need to learn that whenever he's too tired or emotional to go to school, he can have a 'mental health day' off school.*
I have to say I agree with worra though suspect that won't be a popular viewpoint.
If you really did have to let him have today off, you should have told the school why as lying about the reason he's off doesn't set a good precedence to your DS on two fronts: 1) that feeling stressed is something you shouldn't talk about ; and 2) you haven't really tackled the issues which are causing him to feel stressed- school can't even attempt to help if they don't know there's a problem.
I'm not saying mental health isn't important, or that you should always "get on with it" but taking the odd day out for "mental health days" makes something of a mockery of actual mental health problems which are not solved by a day in bed. What you're saying is your tired/stressed and would rather not go in that day. If you really are so stressed that you need a day off, you actually need to address the underlying issue rather than take a day off.