My dad has a severe pain in lower 4 he is diabetic and has a partial denture which gain retention from lower 4 the tooth show mobility so what's this?

Peridontitis?: If the teeth are moble he may have bone loss. Have him see a dentist to determine if this is the case. If he is in severe pain, the tooth may need to be removed.

See your Dentist: The tooth is being over-stressed and taking too much pressure from the partial denture. Sometimes what may look like a simple problem, may actually have more consequences to consider. Each case is different and needs to be assessed by your dentist.