There have been Gedolim who remarried even though they and their second wives were too old to have children. Having children is not the only reason for getting married. I have never heard of any source that said that a man who is too old to have children should not remarry. If you do know of such a source, I would be happy to hear what it is.
But even if there is such a source, there clearly are other opinions as can be seen by the fact that there are Gedolim in such cases who did remarry.