Both are grammatically correct. However, in #1 it would sound better to say "I prepared SOME soup for him". "I prepared him soup" is stylistically weak because "him" can be misinterpreted as the direct object of "prepared". By contrast, "I gave him soup" would be a strong sentence. What's the difference between "give" and "prepare"? With "give", we have a strong expectation for an indirect object, whereas with "prepare" we have no such expectation. That is because the construction GIVE (TO) SOMEBODY SOMETHING is so very common, whereas PREPARE (FOR) SOMEBODY SOMETHING is less common. Grammatically, however, there is no difference.

No, the first sentence "I prepared him soup" is not grammatically incorrect. However, it may sound slightly less common or awkward compared to the second sentence "I prepared soup for him." In the first sentence, "him" is used as an indirect object, indicating the recipient of the action. While it is grammatically acceptable, it is more common to use the preposition "for" to indicate the recipient in this context, as shown in the second sentence. Using the preposition "for" makes the sentence clearer and more idiomatic.

Number two is better.