Oh!  Now you've said he's a mini...  I would have a look see and check that neither testicle is either caught in the band, or has slipped right back through.  If it's important he isn't a rig (with an internal testicle - which, beware, can start to work in a very cold winter - ask me how I know  

), then take him back to the vet for a double check.
Am ringing circa 50 lambs per day and none now or ever have had a leg problem after, so something is wrong ??
Agree.  Up to 10 mins discomfort is not unusual; very rarely up to 30 minutes.  More than that and I would investigate because something is not right.
I castrate a lot of lambs but haven't yet ringed my Shetland x boys as their wee nuts are only every so slightly larger than the hole in the centre of the ring!  I am resigned to possibly having to pay the vet to castrate them later on, because by the time their nuts are large enough I won't be able to catch them!  lol
The only one I've felt happy to ring is the BFL x Shetland - he's about the same size as he'd be if his mum was a Swaledale.  The others, which are more than 50% Shetland and mostly 75%+ primitive / small breeds - no chance!