We have made several attempts to discuss with him and come to some agreement but he has not responded to our overtures.

Aside from that, there is nothing we can do. We have no more ability to force him to stop downvoting you than he does to force us to stop upvoting.

You can take some control and consolidate your SBI upvotes by using #sbi-skip to skip some posts so that more balance accumulates for the posts you allow our votes on. This can reduce the impact of his downvotes (by making them smaller relative to our upvotes). If the relative impact is small enough, then the DV are just a nuisance that we hope will stop eventually.