How to come up with a cute nickname
Hmmm.... Around my area we tend to add a sort of 'azza' extention to last names - for example Jared would become 'Jazza'
However cuteness is subjective - and I can see that not being so cute...
Perhaps something like Jars, or Jar Jar? Maybe if you're a Star Wars fan Jar Jar Binks...?
However these may not feel personal just as any suggested by someone else ... Try thinking of your own by taking a word or two relating to a memorable event you both shared, perhaps a funny one you tease him about or something of that sort.
I would link you to an online nickname generator but those are all rubbish. Here's a tutorial that may help you take the necessary steps to creating a lasting nickname: