It is a mistake. Couldn't help 'thinking' or couldn't help 'but think' would be correct. The first one sounds better, though.

The passage is poorly written, but the only actual mistake I can find is "he couldn't help think.." which needs to read "he couldn't help but ask"... ("But" is essential here. Without "but", it is a mistake.) (since it is a question, "ask" is better than "think") Other improvements: "It was WHILE working there that he noticed ..." "in THE hospital" (or "A" hospital, but you need an article)

"couldn't help" means "that someone cannot stop themselves doing something". it is an odd looking phrase but extremely common.