Mixing it up words

[Pob13]

My DH is 36 and is increasingly mixing up words. For example upstairs instead of downstairs. Really basic things and usually he says the opposite to what he means. He doesnt notice and I either ignore it or correct him. He also has shocking short term memory.
Any ideas what could be causing this?

Covid?
dinething as simple as not using his brain as much, doubt crosswords or word searches can help focus the mind. Scrabble. Maybe. Or cards

I'd get him to see a doctor if it's noticeable and getting worse