It's rare to use 'beautiful' to refer to a man.
We do have the word 'handsome', but it can sound a little old-fashioned, and it also describes a particular 'look'. George Clooney, perhaps?
Younger people don't often use the word 'handsome', and certainly not for people of their own age. In fact, the most useful word to describe a male of any age is 'good-looking'.
You could also use the word 'attractive'. You do have to be careful how you use the word 'attractive', though. Don't tell your friend that you think her boyfriend is attractive - or you might not stay friends for long!