He looks and sounds like a sweet kitten that *wants* to love/trust people ... he just needs incentive to get close enough so he can learn that people are Good.
IMO, any trick you use to help him, and works, is a good one. Give the litterbox idea a try, I actually think that is a brilliant idea as it will help him be familiar with at least one thing if/when he finally does get to come indoors.
I have found tempting food works well for helping kittens to get close enough to be touched and handled. Sometimes even a friendly cat helps, too. I tamed a completely feral adult cat by using a tamed/social former-feral 'buddy' of hers to lure her close to the canned food plate and eventually she became comfortable enough that I would pet him and could then pet her.
The thing with socialization is the kitt/cat cannot become socialized until you are able to handle it. It is getting the kitt to that point of able-to-touch that is the biggest hurdle. This kitten appears to *want* contact with people because he is hanging around, purring, mewing and doing the biscuit dance.
I cannot stress enough: The sooner people are able to socialize him, the better ... especially if your folks want to foster him for a rescue/adoption center. Kitts and cats *must* be well-socialized for public adoption and the sooner he can become socialized and accustomed to handling while going through his appointments for kitten shots and neutering, the more adoptable he will be.
When fostering, the goal is to have the kitt ready for public adoption which occurs after the kitten vaccinations and boosters schedule has been met and then shortly after their spay/neuter appointment.
In this instance, I would strongly recommend your folks trick, trap or do whatever they need to do to get him inside to a 'safe-room' (no where the kitt/cat can retreat beyond arm's length) to begin socialization.
I like to use a bathroom as it is small, contained and easily cleaned. I provide a litterbox, food, water and a 'safe-cave' in the form of a carrier with comfy towel inside and another draped over the top to create a cozy hiding place where the kitt/cat can feel 'safe' and 'hidden' until it feels more confident being in the open with people. In the bathroom, with that safe-cave you can offer tasty food, sit quietly and read, read to the kitty and help it become used to your presence. Food and toys may give opportunities to touch and handle the kitten without frightening it ... and I suspect because of his personality you described ... he will be very easy to socialize once he realizes he can trust people.
He snuggles up to the Big Dog ... he just needs the opportunity to learn he can do that with people, too. I think he's gonna be a great love-kitty and soak up all the attention. Once you've gotten him indoors (do not call organizations until after he is indoors as you do not want to 'flake' and burn bridges by having to cancel if he is accepted into their program but disappears before you can get him indoors) call around to the various rescues and adoption centers and find one that will accept him into their program while he is fostered with your family.