The first thing I learned is that Haiku is not just about syllable count, each word matters and should produce an image or relationship. Here's how I remember being taught back in school:
first line; state a noun, try to use a short descriptive rather than articles
strong striped tiger
or
weeping willow tree
second line; describe an action performed by the subject,showing location can work too, (even inanimate object can 'do ' something), keep it in the present tense if possible.
crouches in long swaying grass
or
offers shade to grassy hill
third line;( now here's where my memory gets foggy) I think I was taught to then pick another object affected by the actions of the first subject, or to pick something that contrasts the first object but is still related. this is actually clarified by the second line, so sometimes you have to work backwards. you could even have him pick two contrasting objects first, then work in the middle line to connect them.
nervous gazelle flees
or
picknickers relax
strong striped tiger
crouches in long swaying grass
nervous gazelle flees
weeping willow tree
offers shade to grassy hill
picknickers relax
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As for critiquing, try to point out extra articles 'the' , 'and', 'a' and suggest replacing them with more descriptive words. crouches in the swaying grass becomes crouches in long swaying grass
It's kind of a difficult form at that age because they have been taught to be careful about using whole sentences. So not using articles, especially at the beginning, is a switch. Tell him to think of it as a picture, his mind does not see 'a tiger', his mind sees 'tiger'. A simple way to describe haiku is word pictures. another tip is to try to get him to pick less common words, to stretch his vocabulary, like i could have used 'knoll' for hill.