In some cases feelings can change, however, a lot of times they do not. Sometimes it just isn't meant to be. It's frustrating to have strong feelings for someone when the feeling isn't mutual. In the end it's up to you; you can stick around to see if he comes around, or you can move on. If you decide to stick around, and the feeling still isn't mutual, waiting around is only going to damage your spirit. You can lead a horse to water but you can't make him drink. In the end, there is always someone out there waiting to love you back 100%!
Sorry for the bad analogy, but feelings can be like food. You might eat something today and not like it, then eat it tomorrow and love it. I would also like to know what you mean by immediate? How long have you known this person, or whoever. Even love at first sight crashes sometimes, then sometimes it works. Just depends really.
I mean at first i wouldnt expect to for them to have deep feelings, that will take time. But the feelings should be mutual. But if his feelings are not where you want them to be in a timely manner, then yes you might need to let him go. And if he comes back, then it might have a chance at a change. Also depending on how old you are, get the book by steve Harvey, that has some great tips on a lot of relationship issues.
In some rare cases, the feelings change and he loves you as much as you love him but in most cases it does not happen. It does not take long for someone to develop feelings and if he does not have them, you may need to move on and find someone else who will return your love.