You probably should not "let it all out" as the solution. If he feels like you're melting down and freaking out on him suddenly it's going to reinforce the idea that he wants to get out of there as fast as possible.

Instead you should definitely talk to him. You should do that before the next play session. Gather some resources about sub drop, send him links, and ask him to read them. Tell him it's really important to you that he understands about these things. Make clear that while you love playing with him and enjoy more sessions that you really need the time after the play and that it's probably not a good idea to get together for a session when he's in a rush. Work with him to find times when you'll both have more time you can dedicate to one another in a block.

After that, be firm. If he doesn't respect your needs then don't schedule play sessions. If you go through this process, he says he respects your needs, and then leaves after anyway, then you know what he really thinks and you can make your relationship decision from there. Hopefully though you can help him understand what you need, and he'll want to work with you to give you that.

Hope that helps.