Hi Erin
Do you actually need him to stay at the moment??? It took me ages for Freddy to manage this and even now it is his weakest training spot, any insecurity has him creeping or all out flinging himself to me. When he is tethered, say when we are camping, he needs to be on a metal cable as he would chew to get to me.
I rarely actually need him to formally stay so I have tended not to bother too much. He can (after 4 yrs of training for it) do a 2 min stay with 30 secs out of sight but he has to be totally confident with the dogs and people in the group.
Conversely I can leave him on his bed, a different room etc for ages now (just by leaving im not commanding him) so it is not SA, he is just uncomfortable and made anxious by being left in a stay

.