He has too much fat _____ him; he needs to go a diet.

人的脂肪在体内而不是在身体表面接触，因此介词应该用in而不是on。不能说go a diet, 而应说go on a diet.

蒋老师说的有道理。

从医学角度讲，人和动物体内脂肪的作用有二，储存能量和起绝缘作用。寒冷地区的动物皮下脂肪层和皮毛共同保持其体温不受冷空气影响。脂肪的储存主要在皮下组织和内脏（如体内的大网膜）。当从外部观察主要看到的是皮下脂肪。因此在语言形成过程中，描述动物的脂肪时，多指体表的松软的组织（即脂肪在人体的表面层）。这样就形成了fat on sb或fat on sb's body.

语料库中有很多fat on him的例句，却没有fat in him. 因此，这个句子中的介词应为fat on him。

He has too much fat on him; he needs to go on a diet.

请问：这里应该什么介词？用 on 还是 in？

与人或者物体表面接触需要用介词on

(1)带在…身上

I didn't have any money on me.  

我身上一分钱也没有。

Have you a match on you?
你身上有火柴吗?

I have those numbers, but not on me at the moment, they're at home.  

我有这些号码，但眼下没带在身上，搁在家里了。

(2)（视线）落在…身上

Everyone's eyes were fixed on him..

每个人都盯着他看。

It's as if all eyes are focused on me. 

好像所有的目光都聚集在我身上似的。

(3)（某种表情）在（脸）上 

The maid looked at him, a nervous smile on her face.

女服务员看着他，脸上带着一丝局促不安的微笑。

She looked at him with a hurt expression on her face.  

她望着他，脸上流露出受伤的表情。

固定搭配用法：go on a diet用规定食谱，节食

"He'll have to go on a diet," Ballard quipped.  

“他得减肥啦，”巴拉德俏皮地说。

Yes. You should go on a diet.  

是的,您应当节食.

 You must go on a diet because you are too fat.  

你必须限定饮食因为你太胖了.

The doctor says I've got to go on a diet.  

医生说我得节食.

Things got so bad recently that he decided to go on a diet.  

近来情况变得如此糟糕,以致他决定节食了.

 I think you shall go on a diet.  

我看尔应该节食.(或:我看你应该吃规定的饮食. )

 I am a little overweight. I should go on a diet.  

我有点过胖了, 得节食才行.

Many people, especially young women go on a diet to get slim.  

许多人, 特别是年青女子,为变得苗条而节食.

Maggie: He advises her to go on a diet.  

玛吉: 医生建议她节食.

I've gained much weight recently. I have to go on a diet.  

最近胖了许多, 我得节食了.

 The doctor ordered him to go on a diet to lose weight.  

医生要他按规定进食来减肥.

He has too much fat in him; he needs to go a diet.

不能说go a diet, 而应说go on a diet.例如：

She is going on a diet. 她正在节食。（摘自《英汉多功能词典》新编升级版）