As a result,he put on so much weight that he found it diffic

As a result,状语
he 主语
put on 谓语
so much weight 宾语
that 引导了宾语从句.而这里的that跟前面的so连接在一起,组成了so...that的句型,意思是,如此,以至于.
he 从句主语
found 从句谓语
it 形式宾语
difficult 宾补
to climb the stairs 真正宾语
这里是常见的句型：
find + it (形式宾语) +adj.(宾补) + to do sth.（真正宾语）