Yes, that will brighten his day up
Say hey I know things are rough now but toΒ just let you know how I feel about you.
My fiancee's mother is in ICU and its his birthday. Should I wish him happy birthday?
Absolutely! He needs the distraction. Having a family member in ICU is stressful, wish him a happy birthday, get him a card, make him a cake...tell him you love him and are trying to alleviate some of his stress, if only for a day.