I want to start by pointing out that texting and calling all day isn't the serious phase - it's the early infatuation stage. Once the serious stage sets in, all that "can't stop thinking about you" dies off a bit and it all becomes a lot more low key - but also solid.
I can't tell you if his pulling away is truly due to him being so busy - or if this is his 'normal' once the infatuation dies down. I guess all you can do is talk with him about it in a non-pushy way and see what comes of it.
Never regret anything that has happened in your life. It cannot be changed, forgotten or undone. So, take it as a lesson learned and move on.