I would think it depends on the nature of penning in your abbattoir. If he will be penned out of sight and sound of other sheep (as primitives are in our abbattoir) and if he will be waiting overnight or until all the commercials have gone through, then I would not take him on his own. If though he would be penned next to other sheep while he waits, then that would be fine.
I am a bit surprised that he will be big enough at 15 months, as a Soay, to fit the machinery in an abbattoir, so check with them before you send him off. If you could keep him until August his hormones would be down still but he would have put on summer weight. Can you keep him with others but where you don't have contact with him while you wait?