My friend has a birthday. I don't know what to say to him.

I ussually text someone "Isnt it your birthday today?" then they say yes then I say "Happy birthday!"

I always ask first because even though I know its their birthday, I worry that maybe it changed somehow lol 

This is an interesting option, thanks But how do I choose words so as not to offend? I couldn't find anything on the internet.

Firstly have you had experience of him being offended by being wished well? I struggle to see how a positive birthday message could offend, so there must be a back story as to why you are concerned? 

Let us know the outcome. 

Let us know the outcome.