An invoice is not hard to write up at all. Wonder what the delay is? I wonder about setting up an agreement that when you give him the moolah, he gives you an invoice. It'd be session by session but for me it'd decrease the anxiety! You need to get some of that money back. That's a place you just don't mess around with people.

I think your T will be apologetic about being so forgetful. Hopefully validate that you need the information and that it was irresponsible for him to leave it hanging.

PS: My boss is ADHD. I use him as an example because being self employed my ADHD boss is very forgetful, when it comes to extremely important things like financial issues. And I've had to sit him down and say I need you to respond to these things because it messes up my life when I don't get things done on time, which means I don't get paid on time. He's trying but still is a struggle.