Does he have any vision issues? Is the floor also white? If so, he's lost the ability to distinguish the bowl from the floor. If you can't get him to sit, place some orange tape around the bowl perimeter to provide contrast. You can get tape at Home Depot or another big box store.
Do you know what has changed that is causing him to miss? Is he unsteady on his feet? Are there grab bars around the toilet? Is he getting dizzy? Does he have a weak stream and gives up trying just as things start to flow?
Men are generally resistant to sitting down to pee, but perhaps you can encourage him to do so. Will be use an portable urinal?
When my husband starting missing the toilet, I just made him sit down on the toilet every time he had to pee, instead of standing. He at that time already had his bedside commode over the toilet, as he needed the handles to help him get up. That did work for quite a while, until he eventually had to get a permanent catheter.
Insist he sit down, and make sure you get a toilet chair with handles and a guard to keep him from peeing between the seat and bowl or elsewhere.
Men are generally resistant to sitting down to pee, but perhaps you can encourage him to do so. Will be use an portable urinal?
A plastic urinal bottle would be his best bet.