Start by trying to figure out why the falling is happening. Is the person rolling out of bed, or getting up and then falling?
If he's rolling out of bed,
the issue is how to keep him in bed while he sleeps. If he's strong and capable of raising and lowering himself safely, one option is to switch to a bed that's low to the floor.
Or consider buying a wheelchair cosy (also sometimes called a Posey, for one of the companies that makes them), which is a sort of half sleeping bag with straps or ties on each side. By attaching these to the bed or the wall, you can restrict the person from moving too far to one side of the bed or the other.
If someone is present in the house at night, use an intercom system so you or a night aide can hear your loved one when he wakes up. Then he can call you to ask for assistance in getting out of bed.
You might also use a hospital bed or bed with side rails, but work with a professional consultant to make sure it's safe. Not many people know it, but side rails on a bed have become complicated in recent years because they've been implicated in numerous injuries and deaths. An older adult can be injured when a leg or arm gets trapped between the mattress or bed frame and the rail. Or a person can become fully trapped, resulting in suffocation or strangulation. Also, if an older adult is capable of climbing over the bed rail to get out of bed, the result can be an even more dangerous fall. Finally, side rails are considered restraints, which are not permitted under Medicaid unless a nurse makes a special case for it. So be careful.
Purchase a newer model bed designed with close-fitting rails and a tightly fitting mattress to minimize gaps between the mattress and the rails.
Keep the side rails in the lowered position, so the older adult has something to hang onto while getting in and out of bed, but they don't impede movement.
Choose a bed that has a headboard or head and foot rails, and work with the older adult to hold onto those when getting in and out of bed.
If the issue is that your loved one is getting up and then falling, be sure to look at the underlying issues. If it's a bathroom problem, talk to the physician to figure out the best way to manage hydration. You don't want to underhydrate, but you can stop a person from drinking past a certain time and then work on a toileting schedule to avoid needing to go at night. If it's an issue about urinary urgency, then the solution might be to manage medications differently or get a bedpan. But talk to the doctor first.
Keep in mind, too, that some medications cause older adults to become dizzy and disoriented when they try to get up. Sleeping medications such as Tylenol PM or sleep aids like Ambien and Lunesta can cause such deep sleep that it's difficult to be fully alert when you need to get up. It's worth checking in with the doctor about those (and all medications).