What is different between ”他一个时候回来“ and ”他一个是后回来“ ？

I think your question is 时候 and 时后?
What is different between “他一个小时候回来” and "他一个小时后回来“ ？

You are correct that "时候" (shíhou) is a common phrase in Chinese, but "时后" is not a standard Chinese expression.

"时候" is commonly used to indicate a specific time or period when something happens or is going to happen. It can also refer to a particular point in time. This phrase is often used in questions, such as "什么时候" (shénme shíhou, "when") or "多长时间的时候" (duō cháng shíjiān de shíhou, "how long"). Here are a few examples:

你什么时候回家？(Nǐ shénme shíhou huí jiā?) - When will you come back home?
明天上午十点的时候我们见面。(Míngtiān shàngwǔ shí diǎn de shíhou wǒmen jiànmiàn.) - Let's meet at ten in the morning tomorrow.

时后, just used with other pre-words, like 一个小时后 (an hour late).

Expressing "after" with 后 or 之后, expressing "before " with 前 or 之前, eg:
一个小时（之）后, (an hour late)
一个小时（之）前, (an hour ago)

Sorry ,what do you wanna to say?
Are you trying to say,he will be back in an hour in the first sentence ,and he will be back after an hour ?
if so,then you can say like this
1:他一个小时就会回来。（he will be back in an hour）
2:他一个小时后回来。（ he will be back after an hour ）

First , your sentence has mistake. both are not right.
it Should be “他一个小时后来回。” means He will be back in an hour.
一个小时= 1 hour
......后= after ........
need to pay attention Chinese grammar order is different English in this sentence.

他 一个 小时 后 回来。
He will come back after an hour.

“小时” not “少时”

后: after

小时候：when you are kids