My husband is prediabetes. will he get dementia?

You should not worry about it at this time. Take care his prediabetic condition by diet and exercise so he won't become grossly diabetic. People with poor controlled sugar level in the long term may develop extensive small vascular disease which can affect the brain function eventually as if you have premature senile dementia. However, Alzheimer disease can happen to anybody, diabetic or not.

Although full-blown diabetes has been said to be one of many dementia risks, prediabetes, under control, would likely be a minor risk, if any, by itself. If he is showing difficulties with cognitive activities, such as handling financial decisions, or getting confused while driving, then he should be evaluated.