So you are a 90 day non-o visa, if you leave and return in those 90 days WITH a reentry permit you visa remains intact.
If you do a 1 yr extension of stay based on your non-o (in country) you would then purchase a multi reentry permit and you are good to come and go as you like for the duration of the extension.
However it only makes sense if you will leave 4 times or more as has been said.
NOW if you want a multi entry visa (from an embassy outside of Thailand) that requires you to leave every 90 days for the duration of the visa, you just leave and apply at the Embassy without a reentry permit which voids your current visa.
I hope that makes sense as your question is a little on the vague side.
If he wants a multiple re-entry permit, to leave and return, that would be stupid to do on 90 days.
He should/could get the single re-entry if he want to leave and comeback in the 90 days.
If he in fact wants a multi-entry Visa obtained at an Embassy outside of Thailand, he would them have to leave every 90 days of the duration of the visa.
2 entirely different things.
Although if the terms not used correctly can be very confusing.