I take care of my mom who is in the later stages of Alzheimer's. I also have a 4 YO, and have had mom and DS together his whole life. Honestly, it has never been a problem, though I can see if your pops is not on the proper meds it could be. Have you had him evaluated for the proper meds? I suggest finding a geriatric psych clinic to take him to to evaluate his meds and reduce the violence. My mom has been to one twice, and it worked wonders. Medicaid pays for a geriatric psych ward too - the entire cost. One big problem I had with mom was her hiding meds in her mouth and then leaving and dropping them on the floor or something - a big hazard for a LO. So you need to check that the meds are actually being taken. There are some amazing drugs out there to eliminate the violence associated with Alzheimer's, but you need to have an expert evaluate them for a week or two to make sure it is right - a regular doc can prescribe them, but they don't know how they will react unless they are in a hospital setting like a geri psyh ward.
As for accidents, my mom is incontenent, so I probably can't offer you advice there - is he in depends? You could get some cloth adult diapers - I tried them for my mom once but she was soaking through them, but it sounds like your pop is not as advances as her, so it might work.
Also, it all kinda depends on pops personality too. Even though my mom does not speak, had no idea who we are or where she is, and is loosing her mobility, she is still the same sweet person. Her spirit is the same. If pop was prone to be nasty to kids pre-alzheimers, I can see it being a problem now.
But seriously, if he is being violent now, I would call a doctor and get him admitted to a geri psych ward now - before the baby comes. The stay is usually about 2 weeks, and when he gets out life will be so much easier. I am all for crunchy natural stuff, but not when it comes to Alzheimer's. I think that anybody who suggests rescue remedy for a person with Alzheimer's has never dealt with the disease on a daily basis.
The manifestations of Alzheimer's (personality wise) vary greatly, so it's hard to predict how your step dad will react to a newborn.
Have you participated in any Alzheimer's support groups? There are a bunch of them around the country, and many online message boards. I bet you'll find better answers there. Good luck!