My stepfather is eating everything, I already know this is part of his dementia however, I would love some suggestions on decreasing this.

Don't leave him access to any food. Have locks installed on the cabinets and the fridge. He eats when you or your mom feed him and that's it.
Believe me you're going to end up with worse than a few mice if you allow this situation with food all over the place to continue. You will get roaches and they are very difficult and very expensive to get rid of.
I've worked in elder homecare for a very long time. I have had many clients who became obsessive/compulsive eaters from dementia. The families had to put locks on the cabinets and on the fridge. When their demented loved ones were no longer allowed to eat 24 hours a day, they adapted to only being fed at meal times only or when someone gave them a snack.
Put locks on the cabinets and the fridge.

Hey everyone, thankyou so much for your suggestions I'm going to try them. I honestly just went into the kitchen to get a little serving of the Blue Bell Egg Nog ice cream I had just gotten and instead found the container in the garbage. I know this sounds silly but I wanted to cry. I never am angry towards him, I just get so frustrated and keep it in. I also wanted to answer that yes he has gained a lot of weight in his stomach area mostly (like a beer belly). I'm living at home with my mother to help her with him, this has been about 3 weeks now. He had a doctor's appointment right before Christmas and he went from a 20 to a 10 mentally in a very short period of time. Thankyou all, I just want to do my best by him and my mother without going crazy myself.

Here is a link to managing overeating with dementia, from the Alzheimer's website:

https://www.alzheimers.org.uk/get-support/daily-living/overeating-dementia

Here is a brief list of suggestions, but there are many more links to click on if you go directly to the site linked above:

Ways to help manage overeating

Ensure that the person has something to do, so that they don’t feel bored or lonely.

Divide the original portion into two and offer the second one if the person asks for more.

Fill most of the plate with salad or vegetables.

Make sure the person is well hydrated as they may be mistaking thirst for hunger. Ensure they have a drink with their meal if possible.

Leave bite-sized fruit or healthy snacks, such as chopped bananas, orange segments or grapes, within reach for the person to snack on when they want to.

Offer the person a low-calorie drink instead of more food.

Consider not having certain foods in the house, or substituting them with low-fat or low-calorie versions. 

If the person has developed a strong preference for particular foods, and is not eating enough of other foods, or if they are struggling with excess weight gain, ask the GP for referral to a dietitian.

I have to say this one more time, thank you all so much from the bottom of my heart. I appreciate it so much, it feels so lonely even knowing so many people are experiencing this horrible disease. It feels less lonely here. ❤

hug!!

sometimes a side-effect of certain medicines is feeling hungry all the time.

sometimes it’s a phase that lasts a few weeks/months, then goes away.

we locked things up.
we also got rid of certain medicines.

the hungry phase lasted a few weeks.

normal and good appetite now.

My MIL went from independent living to AL a couple months ago. For about six months now, we keep her stocked up on single serve puddings. You can get 4 for .87 cents at Walmart on the snack ailse. Refrigerated ones are more expensive. Sam's had a 30 ct for $5.87.. Comes to .20 cents a serving. She could live on these and pretty much does. She throws so much food away at AL, but will always accept a pudding. She will get up during the night and take two back to bed with her. All she cares about at this point is sweets and this solves the issue.

It is my understanding that the disease of Dementia causes large calorie burns and that some patients lose weight. If your father is not gaining weight let him eat. My husband is not that bad but noticed he eats more ice cream and drinks more milk which is fine. Our doc wants him to eat as he has lost weight. I get it with cost of groceries. Talk with your medical doctor and see what he/she suggests.

My father is in a wheelchair, but he would be in the fridge if he were able. He has no short term memory and he doesn't know that he just ate. I think eating is the only pleasure he still gets that doesn't require understanding things (like TV programs) or remembering anything.

Is this mostly snacking, or eating real food?
If the latter, is there something he loves, that is "relatively inexpensive" make, that can be put in separate containers in fridge for him to grab and heat? Chili? Lasagna? Beans with chips? Something he can help you prepare for him?
It is hard in these tough times when things are so expensive to come up with much other than popcorn for snacks, and the digestive systems of elders (mine included) often aren't receptive to some of that stuff.
I sure wish you luck. Prezels from costco, as well as tortilla chips come in massive bags for a few bucks. I sure hope others have ideas for you.

i would leave only healthy food available, like vegetables (carrot sticks, celery sticks), fruit (apples. oranges, grapes), fruit roles, yogurt, protein bars with zero sugar for the between each meal craving. this way this extra food will not be so fat/sugar sustaining and therefore help him get rid of the belly fat and provide healthy nutrition. I suppose he doesn't get exercise either? You have to be his brain. The extra food that i recommend will give him good nourishment, good intestine health and help restore a healthy mind and nervous system. it's really very easy. keep everything else locked up.