Toddler screams “right now!” constantly. Any tips on how to curb this?

I have a 2.5 year old who is going through a phase where he screeches “right now!” about the things he wants, usually food related. It’s usually either right before or in the middle of a tantrum.

What’s a good response to these remarks/behavior? I tell him he’s not allowed to yell, but the phrase is rude too.

How do I explain that it’s rude and suggest a different way to express himself? (Basically how do you breakdown the concept of rude to someone so young + give an alternative way to express himself?)

I’m sure the answer is obvious and I’m too tired to see it.