I think you have to say Did you decide what to eat ? or Have you decided what to eat ? because he shouldn't take so much time deciding it's not a continous action NB ; i'm not a native speaker, i'm just trying to help

I would say 'Have you decided what to eat?'

Do you decide what to eat? is not the correct way you would ask somebody. You want to ask the past tense. Did you decide what to eat? Or have you decided what to eat. This sentence is also past tense. OR you could say Do you want to decide what to eat? but that would be said before someone has looked at a menu and you're are showing them one. I hope this helps :)