He said he's been really busy... is he interested?

It could be anything. It could be kids, family issues, or he is genuinely busy. He is obviously experienced in being called on what appears to be his lack of interest. Or he wouldn't have been so quick to respond. I would say you have demonstrated an interest; he hasn't responded. Maybe sending him a 'thank you' card for the wonderful time you had may jog his memory and interest. When a woman doesn't respond to me after a date, I write it off as just another false alarm. Am I supposed to call? Don't get your hopes up too high. They hurt coming down. Move on with your life. Hold on to what little self-confidence you have left to open up to a new person. This ones knows you are just a call away.