How do ask him for his number?

talk to him like normal, then just say calmly, that you enjoyed the convo, and would like to continue it sometime on the phone, and if he wants to give it to you, he will, then

just talk to him like normal and say at some point or another hey I want to text you latter can I have you number or something.
hey it work on me.

I agree with Monica777 give him your number if you want him to make the first txt / call.
But if you want his number -shifty eyes- this is what I tell my friends to do:
Start a conversation or whatever on something he likes / interested in but you don't know much in [pretending or not] but want to get into [again act or actually am interested] and when you have to leave say- Hey can I get your number? I might text you about -subject- later.

What I do is just give them my number and see if they use it. But if you don't want to scare them to trick them into getting it- for example start a conversation that he keeps wanting to talk about but then say something like "I have to go, maybe we can talk later? here's my phone number..." works for me!