Allergic Testing Help

Our son has floppy airways, so we must thicken his feed. We have used
both Oatmeal and Rice with extremely poor results. We have been on
Neocate for over a month and recently had to go on an NG feeding tube.
He does great when he is fed through his tube with pure Neocate, but
when we feed him with the thickened feed he goes absolutely nuts during
the feeds and for hours after.

1.
Does anyone have information on what exact tests to ask for when it
comes to food allergies? How do they test it? What are the specific
tests called? Are there a wide variety of allergic tests?
2. Does
anyone have any other thickening agent for kids that will be completely
safe and without any allergic agents (I was told before to use
SimplyThick, so I will try that tomorrow hard to find)?

Thank you to anyone who helps and/or responds