If he was rung/ringed with a testicle still up, he 'should' be sterile as the sperm is too hot inside the body. I was reading an article on 'empty sack castrating'. However he will still have very ram like behaviour as he will still produce boy hormones
PLUS - May 2010, several farms hereabouts had unexpected lambs dropping left, right and centre.
We had had an unknown rig running with our keeping ewe lambs, a neighbour used 'teasers' (rigs) to get their girls cycling, and then left the teasers running with the ewes when the tups were removed.
December 15th 2009, a really cold snap began. Temperatures routinely -10 to -15, down to -21 briefly. Apparently, at those external temperatures, those internal testicles suddenly start working!
So my advice would be - eat him. Or if you really love him and can't bear to do that, then get his remaining tube tied or testicle removed.