How do i get my 3 year old son to sleep in his own bed all night?

Question

my 3 year old wont stay in his bed all night we pt him to bed like between 900pm and 1000pm and about 12 or 1230 he would get up and come in our room. i try to put him back but he starts to cry and wont stop he told me his back hurts him and sometimes hes scared hes been sleepin in our bed since he was born our fault. can anyone relate to this and if so how do i get him to stay in his bed