What do I do with the student who has good ideas but writes badly?

First of all, it should be pointed out where the badness lies:

• Grammar
• Structure

In case of grammar, he should be provided with syntax base teaching. For example:

• Pronouns (especially: personal pronouns)
• the combination of:  S(ubject) V(erb) and O(bject)
• Construction of Noun and verb
• Placements of Adverb and Adjectives
• A little grammar review (like tense and active passive)

In case of structural badness/error, he should follow:

• Introduction, body, and conclusion pattern
• Connectives or transition signals
• A paragraph should have: Topic sentence, supporting sentences and concluding line if needed
• There should be consistency in case of using pronouns, especially. (because some students use: he, then she, then you, then they etc)
• If there is a fear of ambiguity creation then main noun should be repeated
• No new idea in the conclusion paragraph of the text