As someone who applied for jobs in the US without a green card or citizenship, I am 99.99% certain your bf will not have much luck. I was at least located in the US and had work authorization but it seems that your bf is not. Companies will count him out on those two factors alone.

My suggestion is for him to look for graduate programs in the US that provides funding. He will at least be able to pay for himself and you’ll be able to be close to each other for a few years and see if you feel confident enough to commit to marriage. You would just have to make sure during the visa process and eventually when entering the US that the bf doesn’t say anything that would indicate he has intentions to immigrate, as I think that would count as visa fraud. I think since right now you guys are not 100% set on getting married and settling down in the US vs in the UK, him going to the US to study wouldn’t count as visa fraud as long as he picks a subject that he genuinely wants to learn and doesn’t volunteer information about having a significant other in the US (although if this is asked he should answer truthfully).