I do not think he will ever fall out of love, no. I do not think he will ever stop loving Mamoru. Mamoru had left a big impact onto Radu, allowing him to embrace his immortality and that loving wouldn't only leave him solemn and in pain caused by his past lovers leaving him behind.
It's a bittersweet feeling, loving people who would eventually "leave" you behind. But the undeniable joyful and sweet times you spent together aren't lost in the sea of doubt and sorrow. Radu remembered them all with the help of Mamoru, proving that although clouded by judgement of poignant hopelessness, he was blessed with people who loved him with all their heart, even until their last breath.
Mamoru's so kind, he has such a beautiful soul. He willed himself to be somebody Radu can depend on. To love wholeheartedly and somebody to smile with. A lifetime worth of pure love is worth an infinite lifetimes of eternal loving. I think they are each other's "forever" :)
Also the fact that Mamoru managed to find a way to spend a hopeful forever with Radu whenever he dreams... that's true love. I love them. I want more... Aaaaaaaaaa sniffles