I've had many many paras over the years. Some paid more attention to the job, some less. Would someone pulling out the phone to send a quick text to check on a kid at home or something like that bother me? No. I'm faily laid back. Would it bother me if it was all day, every day? Yes.
Now, one of the things I learned quickly as a beginning teacher was how to manage the other adults in my room. Being in special education with kids with significant impairments, I've had up to 7 paras in my room at once as well as push in speech, OT, etc. I learned to give direction, even when it was uncomfortable. If you're this person's supervisor you should feel comfortable setting a schedule and encouraging him to stick to it. Maybe have a to-do list at the beginning of the day 1) help J with math assignments, 2) check homework books, etc. You can always couch it in your need to have things run well in your room, you wanting to get a good eval, etc. I often used to start with "I know I am so crazy about this, but I'd like you to..."
Good luck! It's a tricky thing, but something that will serve you well as a teacher if you can get good at it.