The words mother and son both use the sound /ʌ/.
But Mom and Mum have different vowels, thus the different spelling.
And the pronunciation of "mom" depends on where you're from. I have two friends from the Midlands who pronounce it entirely phonetically. It rhymes with "Tom".
If you have a question about the English language and would like to ask one of our many English teachers and language experts, please click the button below to let us know: