I have to say, I don't necessarily see the angle of involving him. I mean, you could, sure, play together with a toy, but if we are strictly talking about alone time, I don't think that should be a problem.
It is an interesting debate, because technically, there's nothing to ask permission for here, nothing you'd have to check in with him. I think it's just an awkward duality of the situation - no reason to involve him in the decision of getting one, but at the same time, by nature of him not knowing about it, it just creates this feeling of being secretive, hiding something.
I think the solution is somewhere in between the two different sides: If he is aware that you have a toy in the house, tucked away in your drawers, it's still something that is yours only, you use it when you feel like it alone, but he wouldn't get the surprise in case he sees it one way or another - Don't know, looking for socks, or something. There would be a significant difference between him knowing what is in the box and literally just keep doing what he was doing, not taking a second look, versus him bumping into something he has no idea about... And just to reiterate, this is not about "reporting", just...clarity? Just trying to see what could work in your situation.