Jim is concerned that Will's experiences on the rock will ultimately make him think he's going crazy.
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It's a fine line in various genres (in particular with fantasy) as to what might be perceived as crazy or not. If you have Will in a relatively staid existence and then throw in some time travel hallucinations, then yes, he might think he's hanging on by a thread.
I'm not sure how far with the hallucinations you want to go, but if it's ultimately part of a fragmented memory coming back to a whole for Will, then you might want to characterize them as repressed memories rather than hallucinations. If he has the feeling that these events happened to him, then I believe reader swho react better to repressed memories than hallucinations.
Hallucinations, unfortunately, are one of the biggest indicators of a serious condition called schizophrenia (and a couple of others, though this is the biggest). When people read about voices or delusions, they think "wacko." (And they'd usually be on the right, not-so-politically-correct path.)
Repressed memories are actually rare in my world, and for sure they are very controversial. There's great debate about whether people make them up, or are goaded into remembering things that didn't happen by very suggestive questioning, etc. I'd research this avenue to see if it's something that might be of better use to you than hallucinations.
If I've gotten something off, please set me straight in the comments below. I'm open to questions!