This should be handled case by case and not only looking by "merit sharing" only, you're correct meriting your own alt is merit abuse, but you need to look more deeper, does he got benefit after meriting his own alt? I find nothing.

AFAIK he's never joining any signature campaign, usually most users meriting his own alt to make his alt merit history are high, so his alt will high likely to get accepted.
He's also not a scammer and high likely wouldn't become a scammer based on his record.