The most important thing, in my opinion, if you're entering this arrangement with Sam is to very clearly--and possibly in writing--spell out your limits. You probably know most of this, but differentiate between your hard limits, i.e. things he can NEVER ask you to do no matter what; and your soft limits, i.e. things you aren't sure you're okay with and/or would prefer not to do, but are willing to explore under the right circumstances. The reason I say writing them down is a good idea is that over time, and especially when you're in scene, it's easy to forget things. If everything's in writing, you and he won't forget.
Also, be sure to establish safe word(s). Some people just have one safe word that means "stop right now"; some have two, one meaning stop and one meaning "I'm starting to have a hard time with this, let's slow down or back up a bit." A lot of people use the traffic light theme to establish their safe words, with green meaning "all is well", yellow meaning "we need to slow down" and red meaning "stop immediately." It's important to have these be words that you wouldn't normally say during an encounter, but that are easy for both of you to remember.
If you're worried about Sam's reasons for doing this, discuss it with him. Establish why he wants to be your Dom, and why you want to be his sub. Make sure you're on the same page as to what you'll both gain from this arrangement. And keep an open, ongoing line of communication so you can tweak, renegotiate, or eliminate all or part of the arrangement.
For me personally, submitting is a benefit because I have a fairly large amount of sexual trauma in my past, with the result that I sometimes micromanage sexual encounters or try to control what my partner's doing. I also have issues around preventing myself from orgasming, not allowing myself to make any sound, and dissociating during sex, all of which are related to the aforementioned traumas. And, because of the trauma and my mother's "education" about sex, there are some sexual activities I can't initiate and, in some cases, can't bring myself to do even if my partner asks me to, because they would make me a "dirty whore"--even though I would *like* to do them.
But... when I'm submitting, I'm pretty much saying to Hubby, "Here's my control, take it, use it with care, and return it when we're finished." Transitioning into submissive mode enables me to let go of trying to control or manage any aspect of what's going on, because Hubby's going to take care of all of it. And when I'm submissive, Hubby can tell me to do some of the things I'm interested in trying but can't do under other circumstances, and I'm able to do them without fear or shame because I'm pleasing my Sir. We've thoroughly and repeatedly discussed, and written, both of our limits (he isn't able to do any pain play with me, for example, because the thought of intentionally inflicting a other than smacking my ass turns his stomach), so when we enter D/s mode, we know what's okay and what isn't. We have a code phrase that indicates we're entering that mode, which can be used by either of us, and a safe word to indicate the end of the scene or a need to stop for a few minutes, which also can be used by either of us.
It isn't something I would recommend trying unless you have complete trust in your would-be Dom. If you feel you can count on him to stick to the agreements and limits, and to see your submission as a gift to be treated with care rather than a "I'm in charge, do what I say or else" sort of thing, it can be beneficial for some to explore.