It's up to you when you marry - whichever works best for your personal situation
. You are allowed to marry in the UK on your student visa, so it shouldn't be a problem to do that.
So, you can marry in the UK before you leave if you like, or you could marry in the US after you have gone back home (but then there's the cost of him flying to the US for the wedding).
If you were able to meet the financial requirement before your student visa expired, you could just apply to switch directly to an FLR(M) visa from inside the UK without having to leave the UK at all (and your student visa status would be extended while it is processing).
However, if you aren't going to be able to meet the requirements until March, you'll obviously need to go back to the US anyway and then apply for the spousal visa to come back when the 6 months for the financial requirement are up.
One possible option:
- Have you been working in the UK at all?
- Have you and/or your boyfriend jointly earned £18,600 in total over the last 12 months?
One way to meet the financial requirement is if he has a new job paying £18,600 but hasn't been earning it for 6 months yet, you can still meet the requirement if he (or both of you jointly) have earned a total of £18,600 in the last 12 months.
If you were able to meet this requirement, you could marry in the UK and switch to FLR(M) in the UK, using Category B of the financial requirement.