What are your options?

a) You do nothing, don't contact him again, don't send him a birthday message. There is only one possible outcome to this scenario: nothing will change. You've lost him and you will be sad. And you will eventually get over it, but you might always keep wondering if it could have worked, maybe even regret not contacting him again.

b) You contact him; starting with just a friendly birthday message if you like. Now here there are more possible outcomes. One is that yes, you were right and he has moved on. He might thank you for your message but say he doesn't want to try to fix things anymore. I don't see it as very likely, but it is possible. But if this is the case, then it is not because you contacted him again, he already felt like that. (Which indeed makes him quite shallow and a person you are better of without.) Contacting him again, even if he's over you, can't do any damage. The other possibilty is that he is happy to hear from you, he's missed you too and he wants to try again. If that's what you want, contacting him is the only way you can get it.

What have you got to lose? You probably are afraid of getting rejected again and I get that. But will you feel better if you don't do anything and just let it go? Yes, in the worst case scenario, it will hurt to get rejected. But you will get over it. And when you do, you can be proud of yourself for confronting your fears!

There is nothing anyone here can do to reassure you. Whatever we say, your brain will come up with a counter argument. You have to be willing to take a risk. If you're not, go buy yourself some ice cream and try to forget about this guy.