When Sweep explores my bed I put toys and things where I don't want him to go and if he decides to climb over those and make his way towards the edge I just put my hands there to stop him or pick him up and move him!
A previous ham I had, Baby Belle, used to LOVE the bed, but she was always so good and never strayed far from me so never went over the edge. I think as long as you watch him he should be ok