If one of the testicles has not dropped (happened to me last year - several tup lambs all by same sire), there is little you can do now...
However the vet can always do a surgical castration later.
I left my boys intact, separated form the ewes in good time and then one of them jumped the fence in November... 5 happy lambs now bouncing about... and yes the second testicle l is now well dropped... they will go away this summer.