A specialist subject of mine is Canine Cognitive Dementia! This is a common condition in dogs as they get older, and can be exacerbated by allowing an elderly dog drift into old age by withdrawing into themselves, becoming less socially active, their reduced mobility and decreased sensory abilities making matters worse as they become more withdrawn.
The first step, which is extraordinarily successful, is to get older dogs onto a diet that can actually reverse the physical causes of dementia. This diet contains omega 3 fatty acid, l-carnitine, vitamin e, reduced fat, and higher protein. Fish, fresh meat, fresh veg, chopped nuts and seeds, yum yum yum! All contain exactly what an older dog needs to not only reduce, but reverse the deposition of substances in the brain that cause dementia. Unfortunately the same can't be done for humans, but dietary intervention, if necessary coupled with appropriate medication, is remarkably successful in older dogs... And perhaps cats too but more research is needed there.
Behaviorally, the older dog needs mental support too. Reduce the length of walks but go out more often, giving the dog lots of opportunity to do social stuff (as long as he enjoys it). So, if he likes other dogs, get him out meeting other dogs. Or kids, or adults, or both, if he loves human attention!
Retrain old skills, like sit, wait, down etc, and teach him some new ones you never got round to when he was young!
If he's not as mobile as before, put his essential resources near his main resting area... Water, food, toys. Buy him some new toys too, and get him using interactive toys like Kongs, Nina Ottosen type toys etc.
Play with him! Hide food, play chasing, do easy fun stuff with him to keep the endorphins flowing!
Reduced mobility can scare older dogs, so help him along by walking up the stairs behind him giving him a bit of support... Ditto for going down stairs and steps. Arrange his bed so he can get in and out easily. Use ramps for getting in and out of cars etc.
If there is sensory loss, eg encroaching blindness or deafness, train him now to a light beam or floor-stomp to get his attention. Place rugs and mats near his bed, food bowl etc so he can feel his way around. Use different scents in different rooms so he can find his way around by smell. Play radios at floor level, it creates echoes so dogs can "hear" where the couch or coffee table is and avoid walking into it. And don't rearrange furniture!
Use a DAP diffuser near his bed so he can both find his bed by scent, and feel chilled when he's in there.
There endeth my rant about older dogs! I know much of this doesn't apply yet to OP's still youthful enough dog, but I hope this arms you to prepare for the future!