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If he is able to see the app in your list of purchases, he should be able to download it for free (it will show a cloud icon with download arrow next to it).
If he’s just browsing the App Store, the correct status won’t always appear on the app screen, but will be recognized if you attempt to ‘buy’ the app (IE A family member already purchased this…download for free…).
Unfortunately, developers don’t have access to App Store account info, so if you are having any trouble with Family Sharing it may be best to reach out to Apple to see if they can find any issues with your/his account.
Of course, if you’re interested, you can get a free year of Infuse Pro when upgrading to Infuse 7. This can also be shared with Family Sharing.