When a native English speaker hears the first sentence, he mentally supplies a missing verb to go with "to him", so it is as if the sentence were actually: [It seemed] to him that the hectic work was both fun and rewarding. The comma empasizes that this is what is going on because it clearly separates "to him" from "fun and rewarding". In the second sentence, "for" works much better. You could use "to" and write "fun and rewarding to him" but if you did that the phrase would again belong with a verb - "was" in this case - rather that with "fun and rewarding". Thus the sentence would be the same as "The hectic work was, to him, both fun and rewarding.

These are two sentences with same meaning, said in two different ways.

It's kind of the same, just another way to say for him.

Meaning is the same it is just different sentence structure requiring different preposition. Similarly…… I am going to give it to you. This is for you. You can also say… I am going to give you it ….if suitable depending on sentence