Hrm. Hadn't realized getting his soulgem did that.
Is there any way to extricate oneself from that to start a different serious relationship?
So unfortunately, no way to back it up (This coming update I added markers to make it clear Mason is a commitment, so hopefully that will help)
However, you can still get his gem without starting his paths. After it all plays out, there's a choice to keep the soul gem but -not- use it on him, in either dominance or submission. Mason loosens up a fair bit and starts learning on his own, and you keep your relationship professional.
I did pull this up and was able to get an option in the bedroom. Like the domme half, it only applies if its the evening (since that is when you tell me to do it). Try heading to the bedroom in your last time period. If that doesn't work, you may have already done something with him that day. Try heading back in the evening the next day to see if it appears. (The requirements on that are that he is in sub mode, he's been ordered to make the call, it's evening, and he's not in recharge mode, which happens after some scenes and goes away during the night)