Hi mate you are doing most things right,is it the ferrets first time,if so they tend to go in then come out again keep at it with the ferret and he will soon get the hang of it.As for him not going down there could be many reasons why, the burrow could be floooded or it could be blocked,did you try him at another hole?
As for clearing the undergrowth before ferreting the burys,yes rabbits will bolt but most will stay put because of the noise above,i normally clear mine the week before that way it gives the rabbits time to settle,then when you go ferreting a week later you just quietly approac