It sounds like your orgasm is the least of your problems with this dynamic, if you genuinely think he might dump you because you orgasmed then I can only say that sounds pretty unhealthy.
Having said that, you may be misjudging him, and perhaps this is an opportunity to see how he reacts. Be honest, tell him what happened, and let his reaction show you how he really feels about you.
There is a fine line between dom, and asshole, trust me I walk it every day. Make sure you get the former, and not the latter.