Both of these are correct. The first of these sentences means that he will definitely arrive sometime in the morning. The second sentence means that he will probably arrive sometime in the morning. (First) I don't know exactly when, but he will arrive sometime in the morning. = definitely. (Second) I don't know exactly when, but he should arrive sometime in the morning. = probably. I hope this helps.

The third one is correct. Based on the start, the person will be arriving in the future, so it can't be the first one. Since you don't know, you can't use 'will'. Then, should can be used since it is not certain, IE you don't know when

你这句话想表达的意思是： 我不知道具体时间，但他应该是在明天早晨的某个时刻到。 整个句子属于一般性陈述，后句想表达未来发生的事情，采用一般将来时。 第二句表达就可以： I don't know exactly when, but he will arrive sometime in the morning. --------------------------------------------------------- 如果在英语学习上遇到什么问题，可以给我留言，非常乐意为大家提供帮助。

我除了第一句不会说之外，后二句没啥毛病。不过还是要看语境。