If he pulls through, yes put him back to mum, but keep an eye on her in case she rejects him. And make sure he suckles, of course.
I am not experienced with injecting glucose, hopefully someone who is will be along soon to advise you on whether the shorter needle will have delivered the solution to where it's needed.
My usual cold lamb solutions may not be a good idea along with the IP glucose, so I won't make any other suggestions apart from keeping him warm, which you are already doing.
for the little guy. They can make amazing recoveries, hope he's one of them.