He misses her like you might miss the stars if the night sky was obscured by an eternal cloud. You'd know they were still there but could never see them. Believing in the existence of something you cannot see is difficult, to say the least.
He misses her like you'd miss a coat in a cruel and cold winter, or as you'd miss a hug when you're upset. He misses her like you might miss oxygen, were it ever taken away....
He carries wounds that won't heal. Stitch them and they hold for a while. But it only takes a little graze or a cut and those wounds tear open painfully and forcefully. They're not terminal, granted. But even when they do begin to heal, they'll certainly leave a lifelong scar. It won't hurt every single day but it may tingle from time to time, like on birthdays, or at Christmas time or even on silent Sunday mornings as she tiptoes carefully around his mind.
He'll ache on special occasions and his achievements will always come accompanied with a bitter sweet "what if?" His first serious girlfriend will ask him questions about her and he'll speak softly and silently, with love. That old scar will tingle, but by then he will associate that tingle with precious memories he'll always have. He might look at the floor, struggling to make eye contact when candidly speaking of how he loved her and his voice will struggle to convey the pain he speaks of when losing her. His girlfriend wlll hold him and she'll probably pity him a little in a way that might momentarily annoy him, before he allows himself to appreciate a gesture not intended as patronising, but intended simply to say, "I can't understand how you feel, but I want to be here if you ever want to talk."
Eventually, he will learn that hugs are people's way of saying that when sometimes the words don't work. He'll learn that people mean well when they ask questions and that it's actually safe to speak of her. He'll learn to predict when the wound might open - most of the time at least. He'll learn, I truly hope, that he needn't push the world out on the bad days and the down days, but instead, to embrace the people he has in his life.