Should I add his phone number? He'd see it?

Hi, I've had a crush on a guy I've known for over 10 years (we're nieghbours and grew up together). He's super cute to me and know he had a crush on me back in the day. Long story short I got his phone number from someone I know and I wonder if I should add him to me contact list (he's in relationship with his cheating ex). (I'm not from the US and in my country there's an app where u can see all the names you've been called in someone's phone list, he has it and he'll for see if I add him name) would I look desparate for adding his number? But on the other hand we talk and we've grown up together. What should I do? Do i add his number or not? by the way his bd is coming up and I wonder if I should congratulate him via whatsapp

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He moved in with his girlfriend wth! What should I do
Should I add his phone number? He'd see it?
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