My husband (diagnosed with dementia) is fully functional physically, but mentally he is very needy. Any suggestions?

There are many excellent resources to learn more about dementia, like Teepa Snow videos on YouTube, and books.

Be patient. It's understandable that he can be short tempered at times, as he is more than likely aware of the mental changes that are going on with him (being forgetful, losing things etc.) and it's scaring him, just like it's scaring you. Put yourself in his shoes. How would you react? Being a caregiver of someone with any of the dementias, requires the patience of a saint (which none of us are, but we do the best we can), and lots of understanding. Hang in there, and come back to vent anytime you need to.