Yup - it's all just pub talk (I'm getting out of this shithole! Gonna make my fortune in America!) if there's no realistic visa path.
Get him to do some actual research, though, because there's no reason you should be cast as Captain Buzzkill when his plans are unrealistic on every level: how you'd legally get out there, how you'd live, how you'd support your dependents. Don't let any 'I had an American dream, but AdoraBell didn't support me so it never happened' bullshit sneak in.
DH and I lived on $50k in California for a while. I absolutely wouldn't recommend it to anyone who isn't a) in their twenties, dependent-free and not bothered about financial security, or b) desperate.
$50k gave us about $3k take-home pay a month, $1500 of which went on rent. Fuel was cheaper; food averaged about the same as the UK; telecomms were more expensive. We lived month-to-month and had no savings. Did have excellent medical insurance through DH's work, which I believe cost his employer more than $1k a month.
I would hate to be broke in the US. It's harsh as fuck. Yes, tons of people do survive on less than $50k, but if they're US-born they may have local family support and be entitled to certain state benefits. They won't have incurred moving or visa costs, and don't need to factor in flights to visit home. They may also live in a way that is completely alien to the average Brit: no accessible healthcare, stuck in an area where gun violence is endemic, working constantly with few employment rights.
I don't mean to make America sound like an apocalyptic wasteland... I like the place. But I hate this blase 'oh, let's just move over there, we don't need much money' bullshit. You're not overreacting! If he's going to be unrealistic, make sure he doesn't get conned into paying for some useless visa-finding service; the kind where they promise lists of employers who'll sponsor you, or instant visas for everyone. 
Send him to the British Expats website - they don't sugarcoat their advice.