Etiquette: persistence vs nagging when networking with senior bankers

I spoke on the phone with an MD recently and he mentioned that he may be interested in bringing me in for an analyst role. He wanted to meet in person and I reached out the next week to organize a coffee but he was busy and then was travelling for business. This was a few weeks ago and I emailed him a couple of times to follow up but haven't heard back.

We had a good rapport when we spoke and I'd definitely be interested in working with him. How much should I keep following up when I'm not getting a response? I know he's very busy, I just don't want to fall off the radar (unless I have already). I've exchanged emails with his assistant but I don't think it's appropriate to reach out to the assistant to see if he'd have time to meet.