How can I get my boyfriend to fall in love with me again?
Short, sad answer: you can’t.
There is no magical set of words or actions anyone can take that will reliably change or alter another person’s thoughts or feelings.
If there is a specific issue that has come between you two - a communication breakdown, a source of resentment, a fading sex life - talk openly with him about what it would take to repair it, what each of you are willing to do, and then work together on that healing project.
But if someone doesn’t love you, there’s not much that can be done besides to graciously receive that information and respond to it as best you can. Do what you need to heal, to take space, to process, and then move forward, knowing that the only person you can control is you.