L.L.
You said he's "perfectly healthy", but (and I am so sorry, my prayers are with you) he is NOT perfectly healthy. Perfectly healthy people can swallow, which is a requirement to survive. If left on his own like this, he would not survive...so I don't call that perfectly healthy.
Even without a diagnosis, I think if they recommend a feeding tube you need to give it a try WHILE they continue to attempt to diagnose. I don't believe caring doctors would recommend an invasive structure into your child if he didn't truly need it. I know it may be hard to hear, but for now, it may be best. Surely, it will be temporary, and they will figure out what's going on.
You don't want him to suffer from pneumonia, either...if it's an ongoing condition, recurring pneumonia could really make him very ill.
I am sorry you are going through this...I will definitely say a prayer for you and your son. I'm sure they will figure it out.