Honestly, there's a lot that Angeal could say in response to Noctis's skepticism. Things like how his presence can't ever interfere with the clear bond between the two men. On what the difference means between giving orders and actually holding the leash.

Little things like that.

But then Prompto gets that collar on, along with the most adorable little ears, and that's it. Noctis eats shit.

Respectfully, despite how very much he wants to give a practical demonstration right now, Angeal just tucks his hands into his pockets and walks lazy circles around the two. "No wonder he was yanking your arm off. You have no control over the situation, do you, Noctis?" Hmmm. If he's being this bristly just by Angeal being around because he made an offer... "Better get a handle on your bad pup, Noctis, before someone properly uses that leash on him."

If his voice does a thing, hey - don't worry about it.

Unless you're Prompto.

In which case, get fucked.

Still, Angeal is hopeful this sort of thing will work. Noctis clearly has a possessive and competitive streak, at least when it comes to the people he loves (Angeal gets it), so a little threat of someone doing better might be the kick in the ass he needs to actually respond... Along with Angeal giving a bit of a hint mixed into the faux-threat.

The leash is there for a reason, after all.