Otherwise, potion of youth would be the easiest in-game way. I haven’t tested that yet (your sim might have to keep having some birthdays in order for everyone else to keep ageing and for you to age him back to young adulthood again), but that’s what I would try if you’re okay with really basic cheats. So theoretically I think you could have everyone else age and just keep editing the age of the sim you want to stay young. I’ve used it before (today actually lol) to age up my toddlers to young adults and just kept the adults as adults lol because I was tired of the kids’ home life and waiting and wanted to just start them out in their adult life story.

You could cheat and use the cas.fulleditmode to modify your sims age which you can do individually.