I think it depends on the context, the situation and the nuance of the speaker. He will do his work. 彼は(彼の)仕事をします。(simply talk about the future) 彼は(彼の)仕事をするでしょう。(with suspicion) 彼は(彼の)仕事をしますよ (仕事をするはずです)。(near to "be supposed to / be sure to", with きっと or ぜったい, the possibility is very near to 100%) I will come and meet you, but first I want to rest. 私はあなたにあいにきます。(simply talk about the future) 私はあなたにあいに来るでしょう。(the speaker isn't very sire if "I" will / will be able to come) 私はあなたにあいにきますよ。(the speaker really has the "will" to come and meet you, with ぜったい, the speaker is 100% sure) I hope this was helpful.

For the first one I would use はず or 訳（わけ）. They mean that you have a strong belief in said thing. for example: あいつはちゃんと仕事をするはずだよ Literally: I have a strong belief/know that he will do his work properly