He has come a long way, and now he wants rest and food.      

这句话没有错误。说 wants a rest 也没有错误，指“一次休息” （让“休息的动作具体化”）。

名词 rest 是一个“可变名词”，即：可数不可数均可。

If you get some rest or have a rest, you do not do anything active for a time. 休息

以下例句为不可数用法。如：

"You're worn out, Laura," he said. "Go home and get some rest."

“你太累了，劳拉，”他说。“回家休息一下吧。”

《柯林斯高阶英汉双解学习词典》

They want undisturbed rest. 

他们想不受打扰地好好休息一下。

《柯林斯高阶英汉双解学习词典》