I think journals are great for personal reasons but of limited benefit educationally, especially with reluctant writers. If he WANTS to do this, by all means encourage him. But you can’t really correct someone’s JOURNAL, and if you push him to do it I can promise you are going to get entries like “Today was nice” “It rained today.” “I can’t think of anything to write today.”
Work on his handwriting for a while. Then start nudging him toward written narrations by asking him to write two sentences and then finish his narration orally. Then gradually increase that. Take your time, writing CAN be learned when needed in not too much time. My current 17yo does well now but wrote nothing of any consequence until he was almost 13.