Hi there
As a vet and long time lamber, maybe I can offer some advice?
If he has a full belly and is warm, he must be getting something from mum, and may not be feeding cos he is full. If his belly wasn't full, or he was looking tucked up, or cold when you put a finger in his mouth, those are signs he may not be feeding properly, and a bottle or a stomach tube may be required.
A full happy lamb will also stretch if you get it up from lying down, where a hungry one will just stand there.
Sounds like he's doing fine in that regard.
As to testicles, sometimes they pop into the body a bit if you are putting the ring on at the wrong angle. I usually sit the lamb on the floor between my legs, with his back legs along the floor, actually in a sitting position. This usually crunches the belly up, so the testicles pop through and into the scrotum properly. If you can feel two then, you should be able to put the ring on. Pop it on the pliers, and stretch it fully open. I usually point one corner towards the tail, one towards the belly, and that way the two teats are along the sides of the square and you won't catch them in the ring. I then put my left hand above the scrotum, to help push the testicles to stay in the right place, and gently close the ring. If there are still two testicles in the scrotum under the ring, then I release it, if not, you can open the pliers and try to refind the testicles. If you are still struggling, perhaps your friendly local farmer could help you out for the first one so you can watch the technique and have a go, as some of them are a bit awkward and try to hang onto them!
Hope that helps
Suzanne