If your only concern is medication then I have nothing to add.
If, however, this is a meat lamb and you are also concerned about meat quality, then anywhere on the back leg is a risk as an abcess can form at the injection site. In that case, top of the rump is best, between the hip bones.
With very young lambs you probably can't find enough muscle to do i/m anywhere but the gigot - but in a teeny one, sub-cutaneous comes to much the same, so in a wee meat lamb, do that.