OK, tell me if this idea is totally mad.
As you know, we've had some problems getting DS to walk independently (to put it mildly) and to cooperate by standing on the buggy board. He is getting a heck of a lot better and manages most Mondays to ride the buggy board almost all the way to and from the CM, for example (it's about a 15 min walk, he gets to run down the hill as the buggy tips with him on the board for that bit!).
I would much rather he walked independently when it's a short distance and it's just with me, or buggy board with DD in the buggy or in her sling (buckles) if I'm taking her out.
DH and I will both take him places in the buggy that are a bit further if it's just him, and I occasionally still put him in the buggy and DD in the sling for a longer walk when he's going to be really tired e.g. after lunch. DH is willing to "just" carry him but I have put my foot down and won't, even if it's just him and me.
But she's getting heavier and so is he (he's 16.5 kg) and sling plus buggy is really hard work, as is a longer walk with just him in the buggy.
I'm getting to the point now!
I would like to mainly be able to go out when it's just him, without the buggy. For example, we go to his dancing class on a Saturday morning and he's the only participant in a buggy if we have to go somewhere else afterwards, though some younger siblings have buggies. It's awkward taking him in it, with steps etc. and he is of course heavy.
I am considering trying a sling for him again. Would be a back carry in buckles, I could rent one before buying.
I am worried though that this will set him back in the walking/buggy board stakes. I would only be prepared to use it if it was just him, not DD.
Part of me says he's still just a baby and he'd like it, and I wouldn't have to take the buggy out with him, part says it would set us back in the cooperation stakes, and part that he would just be really HEAVY.