1. I've never heard of anyone making a lat move to an 03 field from a non-combat arms field.
2. 0321 is not necessarily Force Recon. Could be BN recon, but the above still applies.
3. If he was a SS school graduate, he would have a secondary MOS of 8541.
4. 8652 - Recon w/Jump, 8653 - Recon w/Scuba, 8654 - Recon w/both
He may have been attached to a FR unit in a support position, but I don't think they have linguists. The old rule that the "real deal" guys don't talk is not always true. I have a few buddies who will talk for hours about their service...to another Jarhead. So, don't rule him out just yet.
Start probing for answers. Ask him when he went to BRC. Was he east or left coast? If he says east coast, ask him if he knew a guy named Moose. If he's legit, he will figure out that you are "calling him out" and clear things up quickly. If this guy is actually a Marine and is lying, I don't need to tell you that you have a sacred duty to locate him and beat his ass, but if it turns out he is for real...then you've got a good source of knowledge that you know you can call on in the future. He will not be offended by you being suspicious.