The fact that the sock returned to you gives you an idea. If dad was pleased with his new body, you didn't need to return his excess weight to him, after all, he hadn't been going to the gym and wouldn't get rid of it himself.
With that realization you decided to play along with the diet and exercise program until you can force a sock mix up in the locker room, the socks could easily pass for normal socks if someone wasn't mindful to look for the thin green trim and it only took a few seconds then reality adjusted.
You went with your dad without complaint and began considering things to look out for... such as balding... you'd really like to avoid that if you can. Someone who's fat as you are won't do but you could potentially step down in fatness over a couple swaps. People wearing ankle socks will notice the socks right away and could cause an uncomfortable situation...
Arriving at the gym, it was busy. You got out of the car and although you were overweight, you also gained all of dad's muscle mass and would only be a little overweight if you were your adult height. So grabbing your gym bag and walking to the locker room weren't difficult for you. The weight and body hair just made you look 25 instead of 15.
Most of the gym goers were men. There were a few people who looked your dads age but by far the biggest crowd was those in their late 20's to early thirties. There were a couple handfuls of college students, and 2-3 high schoolers.
He made his way to the locker room and was pleased to see he was following a couple decent candidates, one man looked to be in his 40's with a decent build. Another man, mid 20s, seemed to be a gym addict, lean, ripped, and huge, his muscles looked to be trying to leap out of his skin. Finally there was a man in his 50s who looked huge like the second guy, but he wasn't as lean.