It really depends on the reason that he stepped down.
If it was because of doing something unscriptural, then he may have to work towards being a MS again.
I have been approach several times to become an elder again, since stepping down due to health reasons in the past. I have declined the offer. It's different when you know what you can expect when you enter that service. I'm not against helping in the congregation, but, lately I'm tied down with the same health issues, and work situation that makes the eldership duties hard to manage.
The recent Org book mentions that Elders can step down for personal reasons,and mentions that he should be respected for this decision. The book mentions Retired Elders. These are not Ministerial Servants since they also work very hard, and I find that they work harder than some Elders do.