Does your other furbutts use the door? Maybe if Scooter sees Bitsy use it, he'd be more inclined to use it himself? Or.. try to make it a game using the door? How are you using treats to encourage him? Giving him a treat if he comes through? Leading him with the treat to get through the door? How about giving him treats for being near the door then gradually touching the door with his paw and then to pushing the door?
Sorry for all the ???, I'm just throwing out suggestions.
As for safety... We have big birds flying about from time to time. I live in a corner house so we don't have a back yard. Ended up fencing a part of the patio for them to potty at times. But b/c of those birds and other reasons listed by others above, I specifically supervise any time they're outside.
As safe as it might seem, I don't think it would hurt to take extra percussion... How about some sort of sensor that'll sound an alarm around the fence?