Hi there,
Im sorry to hear about your son feeling unwell.
I had a VP shunt for 16 years, which was fitted when i was 13 months. As of friday just gone, im now a VA shuntee.
When it malfunctioned when i was 4 years old, all tests that were done on me came back fine. However i soon went into a coma, even then, tests came back showing no signs of shunt malfunctions other than the physical signs and symptoms, vomiting, headaches, irritability etc.

In september just gone, i was at Great oromond street to have a routine eye check to see whether this was the cause of the gradual onset of headaches over the years. These tests came back fine, until september, when papilledema showed up. Strange thing being that i hadnt felt better in a long time, yet this is the only time where evidence of intercranial pressure has showed up.
I was fitted with an ICP monitor and after a few hours after surgery, i again showed the symptoms i had when i was 4 years old.
I also then went into the state that i was in back then too, the coma. I was taken to an emergency CT scan, there was no change in the scans or xray series.
Yet it was clear i was having a shunt malfunction.
In surgery the pressure was so so high that when they drilled into my head, fluid flew across the theatre at such power.

I suggest an opthamologist eye test if possible and or ICP monitoring. =)

Good luck to you and your son