Why we always say 'HE HAVE HIS DINNER' instead of 'HE EAT HIS DINNER

You're wrong. We say:- He has his dinner. Or, Does he have his dinner? Or, He doesn't have his dinner. Because it's more appropriate than eat.Eat only mentions "eating". But have/has mentions eating system completely.

We never say 'He have his dinner' . we say 'He has his dinner'. instead of 'He eat his dinner'. n reason is already answered.

When talking about a particular food or meal, eat and have can function interchangeably most of the time. Of the two, have is the more versatile and generic word. For example: I eat breakfast every day at 8 o'clock. I have breakfast every day at 8 o'clock.

Because have is more appropriate than eat