Advice on dealing with blindly bolting horse?

getting a bit worried,,my daughter has 15.2 gelding he is rising 10 but he was backed late at 7,,he is absolute sweetie on the ground but is very nervous when shes in the saddle,he has always almost flinched and his bum goes under him when she is in the saddle can be either in any of the 3 paces,,its like hes spooking at his bum,,she has to work him for good 40mins before he starts to relax and come down on the bit,,even then he can still scoot forwards,,she is learning to relax her hands he doesnt do well with heavy handed ness,,but she finds,,she relaxs he reins but then she feels vunrable,,however just lately he has started to bolt with her,,its very frightning,,,it does un-nervr her she has never come off and always carrys on (doesnt get off and put him away) it just makes me panic it an accident waiting to happen,,had his backed checked teeth done ,,shes only small not that strong when he bolts she has no hope cant even turn him,,he turns off,,,happened the other day when the wind blew,,,any advice please