It depends on what their name is a lot of the time. You could call him something cute that is more personal like maybe something from a memory you both share that will remind him of the time you spent together every time you call him that.
I am a girl, and I think that nicknames like hun, boo, etc. are fine, but not as good as nicknames that are special to the person. My name is Lauren, but I love it when my guy calls me Lo.
depends what is his name is make sure what you choose is not offending, also wait for something funny to come along and go off that
ex:
Tomas= tom tom
if they say something like about a...