@Karladaisy I agree with
@canary, it would take a while for the mirtazapine to make any difference. You posted last Tuesday to say that your granddad had pain in his head and was struggling to breathe and you were waiting for the out of hours doctor to arrive. You also mentioned that he had been given antibiotics last Monday but you didn't think that he had an infection. What did the doctor say? As your granddad is getting worse and is now weak, sleepy and unable to walk, personally I'd call 111 as it sounds like he needs to be thoroughly assessed as clearly something is wrong, and the antibiotics don't seem to be working.