What Is The Difference? There is no difference in meaning. You could say. "He will have (x) and I will have (the) x" OR "He will be having (x) and I will be having (the) x" Or "He'll be having (x) and I'll be having (the) x" or you can add in an optional (the) for each person or leave both out or swap the person. "He will have (the) (x) and I will have x" (the) = optional for one or both people (x) = the thing or things being ordered. will be I think is a relic from our old way of speaking but it is still used. "will + be" = in the future, near future when the food is served. "I will" = in the future, but now at this time is implied (not too much later or tomorrow) this meal/time. Current English is preferring to use the direct short ways of expressing things "i'll have (the) (x)"

Both mean the same thing, and both forms are used here probably for stylistic purposes. It's boring to say the same thing, so the author added some variety.