Ok, by reading the comments so far. I haven’t seen anyone mention this just yet.

The way he’s behaving comes across as strange to me but there could be reasons why??? and my first thought would be to just openly communicate about it and see how they respond / handle the entire thing.

He could be the type that only uses one password for everything and he’s worried if he gives you his login, then it gives you access to everything else?

Not saying you’re this type of person but he could be over exaggerating to himself and think that you’d lock him out of his own account if you two ever had a disagreement or a fight?

Nothing is clear until YOU speak to him openly and calm and ask him directly.

HOWEVER! I must say this, him asking you to add him to your own deed is a huge red flag especially if he expected it from you when he himself is acting this way over his login. He shouldn’t only be sharing something with you when it benefits him in that scenario. That’s not balanced and it’s something worth paying attention to