Fiance and spousal visas are only for US citizens who are getting married to UK citizens - as he is on a Tier 2 visa, I'm assuming he is not a UK citizen?
If this is the case, then you will not be able to apply for a fiance or a spousal visa because you won't be eligible for one.
Instead, if you want to move to the UK with him, you will need to apply for a Tier 2 Dependent visa.
However, In order to qualify for it you either need to already be married (as there's no such thing as a Tier 2 dependent visa for fiances) or have been living together for at least 24 consecutive months before you apply.
I'd say your easiest (and possibly your only) option would be to marry in the US and then apply for the visa afterwards.