I totally agree that the goal is to stay in shape vs "get in shape".
I also think that the best "plan" includes a mix of strength training, aerobic training and hiit training- that's for any age.
I'm in the "old man" group and my "plan" is to hike 4-6 days/week and strength train/hiit train twice a week. I make it a priority getting some longer hikes in- time on your feet translates well into hunting. If it's a backpacking trip with a pack on, all the better.
About two months out from hunting season, I'll start hiking with a weighted pack. I'll vary the weight and the distance, but shoot for two weighted rucks/week. I don't crazy with the weight as the risk of injury outweighs the reward imo- an injury right before hunting season would be the last thing anyone wants. I'll work the weight up to about 70 lbs, but no more. I've never had trouble with heavier pack outs during the season with that approach.
I eat healthy, but far form a fanatic on anything- too many things I enjoy

. I've been at it for awhile, so I can tell when I need to pull things back a bit and recoup/recover. I would suggest including in his "plan" a few rest/recovery days and adjust as he progresses.
My final advice is to have him start right
now, even if it's just walks around the neighborhood. Build the base up slowly (but steadily) and add some strength training in. If he commits to it, you may find yourself trying to keep up with the old man!
