Items and people. Can you train both?
Yes.
For people, given his age, I would start out with easy hide and seek games. Have someone hold him, and you hide - or if he's more attached to some else, have them do the hiding initially. When he's being held, he should be encouraged to watch where they're going, and the person hiding should run and make attention getting sounds to keep him enthusiastic about going off to chase them. Don't run far at first - you don't want him to lose focus on the person running away. Duck behind a tree or on the other side of a car or around the corner of the house and then the person holding him can release him to go chase/find. It might take some encouragement from the person who was holding him, it might take the person hiding calling him so he can be reminded of why he's looking, but it should be simple enough that it doesn't need much help. When he finds you, give him huge rewards.
As he catches on, start going a little further out of sight when hiding. Still let him watch you run off, but after ducking out of sight, go a little further so he has to really search, but not so far that it's discouraging.
Eventually you'll progress to hiding without him watching you run off as well as progressing to having other people do the hiding. But don't try to get there too fast and keep in mind at his age he's going to be easily distracted so keep it simple for a while.
When hiding, consider wind/weather conditions. Different dogs respond differently to the conditions, so it's hard to say what's best. I would start with the wind blowing away from you and towards the dog, but for some dogs that makes too much scent. I also would start with cool weather and a little dampness, but again, that can make for too much scent for some dogs and overwhelm or confuse them. Keep a log of what you do, what the weather conditions are and how he does so you can see what works best. As he progresses you can work on conditions that are more difficult for him, going back to an easier/shorter search to balance out the more difficult conditions.
Also keep in mind what kinds of places he might be asked to do this if it's going to be used for anything other than a game. Woods, water, snow, mountains. Crossing wooden bridges, navigating over fallen trees, slippery ground, searching indoors with a variety of floor surfaces, obstacles, barriers. Ignoring other people/animals/distractions. All are things you would want to incorporate into his day-to-day experiences now so that it's not a big deal when it's needed.
After typing all that out, I previewed it and saw that you'd added to your post about finding dogs...TBH, I don't know of anyone that trains dogs to find both people and other dogs. I don't know how successful it would be to train for both. I also don't know a lot about how dogs are used for finding lost pets but I think a lot of it has to do with the handler having an understanding of lost pet behaviors.