Phlegm/Gelatinous Vomiting

[DeanHUK]

My son is now five years old, but has from a very young age been prone to vomiting frequently after eating/travelling/getting excited/etc.
I have two other children, who obviously have been sick in the past and the process of vomiting itself it not the issue.
With my youngest though the vomit itself is different, it does contain the recently eaten food but not to a great degree, but the bulk of the vomit is of a gelatinous consistency and these is where I think my Phlegm connection is made.
Whether it is undigested phlegm or not I don’t know, or is the stomach juices themselves not right?
My son is smaller than I would have thought for his age, but is generally a happy child with a good appetite (if a bit selective!) So I am worried that he is losing the necessary goodness from his meals because of the frequent vomiting.
I would say that the time period between occurances would be not within two days and can be upto a week.
Hope you can shine some light on this problem.
DeanHUK