We're not sure if dad has dementia but what kind of care is needed?

What causes dementia? Mom and Dad had the worst upbringings. Dad is 12yrs older than mom and has always been kind of an absentee dad, husband, sibling. He's always lacked social skills for some things but was kind of a womanizer. Now, my mom and dad live together as roommates out of necesity to survive financially because they're combined income is less than 15k yr. I'll tell you in a second the reason for this background story. Dad has always been healthy as an ox but that last few years we've noticed his mental health/behavior deteriorating and drastically in the last couple of years. It started with small things like, "dad can you check in the shed for the hammer" and he'd take about 30 minutes and come back inside without anything in hand. When asked about the hammer he would lie (you could tell) and say it wasn't there. Then it progressed to more significant things like, going to the wrong doctors office for his appointments- one he hadn't seen for years. Then, it was not so random for him to not know basic things like cities he worked for years in or highways he traveled for years or cooking somthing basic. Now he is just, isolated (which he had never really been to social with his family), he lacks hygiene (coming from a man that would shower daily if not twice a day), won't even change clothes, he lies, we've caught him stealing (candy bars) at convenience stores and digging thru our purses, he won't leave his room unless it's to use the restroom or smoke, forgets to shut the restroom door, started dragging his feet and inches thru a room dragging his feet an inch at a time, pulls off skin tags and sits there bleeding without telling anyone, he cant drive anymore but when he would (which was up to about 3 months ago) he would go to the grocery store 2 blocks away and bring really random things - especially if my mom requested something in particular he would bring the wrong groceries or the wrong fast food meals written down for him. He has fallen recently, is aggressive with my mom - threatening her life, will hit himself when told not to do something inappropriate, he acts like a child- if guests are over he'll just come out of his room with a funny wig/ hat or fake nose & glasses as if adults and teenagers would think that was funny, or he'll show people he lost his teeth while in the middle of family gathering. I mentioned he doesn't leave his room but if no one is home he'll go and do something incredibly dangerous like unloading 50lb bags of cement (he's 74) off the loaded pickup. He would have the same conversations with me everytime when I would smoke and go outside to smoke with him- same exact things everytime. He's pooped his pants in public. I don't know if this behavior is normal but I know its affecting us. My mom was in the hospital recently because she was just dishonest with hypertension and her BP was out of control and while my sister had me taking care of the kids she was with my mom at the hospital, dad was home alone and fell! My sister came home after mom was discharged and took dad back you the urgent care because he said he had no control over his legs and seeing the video left everyone concerned he might be having a stroke but he was perfectly fine - was it to call attention? My mom and sister are sure frustrated but now even concerned. I feel terrible for thinking this but I think this is his way of sticking it to them because they're short with him. Like I said, my parents are together out of necessity and I've offered to Dad or my mom and relieve both my parents of their miserable lives together but my mom refuses to be a burden on us. My older sister lives with them and pretty much supports them because they would not survive on their income alone and she needs the help as a single mom of 2 kids but that's a whole different story. But to address the background portion- I don't know if this is dad's way of sticking it to them or if her is sick. My fiance says he's just old and is probably unhappy as much as my mom is and this is his way of sticking it to her and my sister but even if that were the case and he just wants to reciprocate the miserable life he has with them, that is still disturbing behaviour in itself! I don't know much about dementia or Alzheimer's but independent of our family disfunctions all around his behaviour is the only one that is not normal - socially unacceptable, DISTURBING! What kind of doctor needs to see him, what kind of tests need to be done? I'm afraid my family is frustrated with him when maybe he should be getting medical attention and if not, he at least needs mental medical attention for his behaviour. I'm to the point where I don't even want a wedding because I'm afraid of what kind of a show hes going to pull or afraid for him to be around my baby that's on the way. I hate not knowing how to solve this problem. Despite everything - we are a very close family and his behaviour affects everyone in so many ways! Please help.