There are a few ways to keep older dogs who are less inclined to play happy and invested in a more natural need satisfaction - we have a Shetland sheepdog and he'll likely never herd sheep but he gets occupied with other activities.
Firstly we take him geocaching a lot - this involves plenty of walking but he's gotten very used to helping find the geocaches as they're usually all plastic and start to smell the same (even to a human nose they smell like musty damp paper etc). This isn't a trail but when we're in the right area and start looking around he loves hunting around in the undergrowth as much as we do!
You can purchase animal scents online and do a trail in your own garden every now and again if it's big enough, hiding a good treat or toy at the end of it (fox urine is a common purchase in the UK as it's replaced actual fox hunts). Though urine scents will linger, so to create a fresh trail it'll be best to either water your garden or wait until a dry spell after some rain. You could feasibly do this with a really smelly treat instead of a scent but I'm not sure how well it would work (he might cut a few corners if it's too pungent?)
You could take a dog to agility, it's mentally stimulating as the dogs have to perform specific tasks. Usually you'd start this at a year old (you should wait for complete bone development before starting on this) but old dogs can learn new tricks.
James Jenkins mentioned the Barn Hunts and that's also a great other options and the link he gave gives a brilliant answer to how that works: What is a "Barn Hunt"?
Another option is to possibly take him hunt training, you don't actually have to take him hunting itself. My sister has a gun dog whose currently a young puppy and she's due to go to training as a retriever for pheasants. There are courses about if you can find one near you - you may find he enjoys it and you can possibly spark some ideas for how to entertain him better based on this.