Ah OP, I think maybe you aren't always aware that you treat him like he's much younger than he sees himself. Some examples below.
You are hoping his relationship doesn't work out so he can come home - but you have no idea if it's a bad relationship and so it's quite harsh to hope he'll end up heart broken just because you want him back home. Even if it doesn't work out, hopefully he'll have a taste of independence and not want to come home. And you can be proud he wants to do that - shows maturity. But if he can sense this negative attitude, he'll be reluctant to include you in the relationship.
You don't think he's mature enough atm to deal with living alone/paying bills etc - at 21 maybe he thinks it's time he learnt and the only way he can do this is by moving away. It's possible he's seen his gf's family set up and thought it will offer him more freedom. And he thinks you'll never be happy with him moving away until you feel he's ready - except he's ready now.
You have tried to find where his gf lives instead of respecting their right to privacy as adults. You're doing it out of concern as a parent but at 21 he won't see it that way. If he or she had seen you driving by, it would have convinced him he was right that you are too involved in his decision making and life.
Similarly with restricting access on FB. It's just a sign he wants privacy to have his relationship without the opinions of his parents. Until he's ready to talk about it with you. The fact he is still sharing with his siblings shows it's a boundary he only has with parents, not the entire family. So that's good because it means he's not alienating himself from everyone. Most young men are coy about relationships with their mums - it's not personal.
So when you do meet him, don't make him feel guilty about the decision to move away and not tell you. Reassure him that you will support him no matter what and would like to meet his gf, but only when he's ready. For some reason he has this idea that you will get upset about him moving away no matter what - so if he can see you be calm and happy for him, he'll feel more comfortable opening up. 21 is that age where he's figuring out his own place in the world, craving freedom etc and it is hard to do around family - so it's just an adjustment for everyone.