"Can I just enter it as "Other Income" on Schedule 1"

That's not where Pensions get listed.

"I have no EIN to put into the worksheet to avoid errors"

I though you told us it is Corporate owned. Somewhere there is a plan document, a check was issued, some sort of document explaining to your taxpayer his rights and coverage. I don't know if this requires an Annual Plan Summary, a Plan Document for those covered, there must be a Contract or annuity.

Make your taxpayer get hold of the ex-employer or dig through their papers.