The heat made him feel he wasn't up to more.热使他感到再也受不了了。

Up to doing sth 表示某人胜任做某事。用在系动词之后，常跟有-ing小句。

例：
If you don’t feel up to writing it along，we can do it together.
如果你感到独自写不了，我们可以一起写。