I think it a good idea to ask him some advice.对吗

kdxn回答的基本上正确
应该是
I think it's a good idea to ask him for some advice.

The following two sentences are right:

I think it is a good idea to ask him for some advice.
I think it a good idea to ask him for some advice.

错:
I think it is a good idea to ask him some advices

I think it's a good idea to ask him for some advices.

ask for

I think it is a good idea to ask him for some advice.

书面语里避免用缩略结构it's

ask for 是固定搭配，向某人要求什么

advice 是不可数名词

I think it is a good idea to ask him some advice.