I think you need to be careful. You could drive him away (and create a rift between you and his partner) with "gifts" like fit bits and gym memberships. Someone who decides to lose weight does not need those things -- it can be done without them, and can only be done when they decide it is a priority. I'd serve and encourage healthy eating when you are with them, and something active together walks or hiking or something) when you are with them. You may feel you have to talk to him about it. I would do it ONCE (not once per visit or per year). Then I'd stop. He is a sovereign adult, and will just stop coming around if you talk about it.
It isn't restricted to just HS, but it is rare that an experience in middle school is something they really want to hear about. This is the one time when answering the prompt isn't really what they are looking for. Know that college admissions staff reading your essays didn't write the prompts. I'd look for something more recent. If you haven't read these yet, check them out: