Hi! Did he always enjoy walking and exercising? What is his reason for not wanting to go? I wonder if you said that you were “training for a marathon” or “trying to work out more often” and suggested that you needed HIS help to do so, if he would go. Typically, if you ask for help, another person may find it harder to turn down then if you just try to get him to do it for his own health. He may feel honored that you’re asking for his assistance with your “training.”
On a side note, make sure you are with him (or someone else is) while walking outside because he could get lost. Also, do you mean that he is in the early stages of dementia, or is he under the age of 65? Early-onset means that somebody is pretty young, like well under 65, and the symptoms move a little quicker.