As-salamu alaykum Ustadh, احسن الله اليكم my late grandfather’s half-brother (they shared the same father but had different mothers) will be visiting our home and staying for few days. Is he considered my mahram, or should I cover myself and wear the niqab in his presence? JazakAllahu khayran.

Ustadh AbdulAziz al-Haqqan

Yes because he is your father's uncle, so he is a mahram for you.