These two are usually opposite conditions, so I think you're going to need to get some clinical advice on his specific case.
Impacted stools from severe constipation can also cause diarrhea, as that's all that can leak past. This might need a scan to identify. This can be caused by dehydration and/or lack of fibre in diet, but may need more than just that to resolve. Constipation to this degree can cause delirium too. Many older people don't drink enough fluids, particularly if they have any urinary issues.